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HYDROMEDUSAE

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Originally appearing in Volume V14, Page 147 of the 1911 Encyclopedia Britannica.

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HYDROMEDUSAE , a See also:

GROUP

group of marine animals, recognized as belonging to the See also:

HYDROZOA

Hydrozoa (q.v.)by the followingcharatters . (I) The See also:

POLYP

polyp (hydropolyp) is of See also:

SIMPLE

simple structure, typically much longer than broad, without ectodermal See also:

OESOPHAGUS (Gr. o'lvca=I will carry, and r/)ayeiv, to eat)

oesophagus or mesenteries, such as are seen in the anthopolyp (see See also:

ARTICLE (from Lat. articulus, a joint)

article AxrHozoA) the mouth is usually raised above the peristome on a See also:

SHORT, FRANCIS JOB (1857– )

short conical See also:

ELEVATION

elevation or hypostome; the ectoderm is without See also:

CILIA (plural of Lat. cilium, eyelash)

cilia . (2) With very few exceptions, the polyp is not the only type of individual that occurs, but alternates in the See also:

LIFE

life-See also:

cycle of a given See also:

SPECIES

species, with a distinct type, the See also:

MEDUSA

medusa (q.v.), while in other cases the polyp-See also:

STAGE (Fr. 6/age; from Lat. stare, to stand)

stage may be absent altogether, so that only medusa-individuals occur in the life-cycle . The Hydromedusae represent, therefore; a sub-class of the Hydrozoa . The only other sub-class is the Seyphomedusae (q.v.) . The Hydromedusae contrast with the See also:

SCYPHOMEDUSAE

Scyphomedusae in the following points . (I) The polyp, when See also:

PRESENT

present, is without the strongly See also:

DEVELOPED

developed See also:

LONGITUDINAL

longitudinal retractor muscles, forming ridges (taeniolae) projecting into the See also:

DIGESTIVE

digestive cavity, seen in the scyphistoma or scyphopolyp . (2) The medusa, when present, has a velum and is hence said to be craspedote; the See also:

NERVOUS

nervous See also:

SYSTEM

system forms two continuous rings See also:

RUNNING

running above and below the velum; the margin of the See also:

UMBRELLA

umbrella is' not lobed (except in Narcomedusae) but entire; there are characteristic See also:

DIFFERENCES, CALCULUS OF (Theory of Finite Differences)

differences in the sense-See also:

ORGANS

organs (see below, and SCYPHOMEDUSAE) ; and gastral filaments (phacellae), subgenital pits, &c., are absent, (3) The gonads, • whether formed in the polyp or the medusa, are developed in the ectoderm . The Hydromedusae See also:

FORM (Lat. forma)

form a widespread, dominant and-'highly differentiated group of animals, typically marine, and 'found in all seas and in all zones of marine life . F1'ek,h-See also:

WATER

water forms, however, are also known, very few as regards''speeiss" or genera, but often extremely abundant as individuals . I1f• the See also:

BRITISH

British fresh-water See also:

FAUNA

fauna only two genera, See also:

Hydra airs See also:

Cor,dylophora, are found; in See also:

AMERICA

America occurs an additional getl$, Microhydra . The paucity of fresh-water forms contrasts shar$ly with the See also:

GREAT

great abundance of marine genera See also:

COMMON

common in all See also:

sea's, and on every See also:

SHORE

shore .

The species of Hydra, however, are extremely common and See also:

FAMILIAR (through the Fr. familier, from Lat. familiaris, of or belonging to the familia, family)

familiar inhabitants of ponds and ditches . In fresh-water Hydromedusae the life-cycle i ,usually secondarily simplified, but in marine forms the lift-cycle may be extremely complicated, and a given species often passes in the course of its See also:

HISTORY

history through widely different forma adapted to different habitats and modes of life . Apart from 'k€ral : or embryonic forms there are found typically tw`o types of See also:

PERSON, OFFENCES AGAINST THE

person, as already stated, the polyp and the medusa; each of which may vary independently of the other, since their envrotl`ment and life-conditions are usually quite different . Hence both polyp, and medusa present characters for See also:

CLASSIFICATION (Lat. classis, a class, probably from the root cal-, cla-, as in Gr. icaMce, clamor)

classification, and a given species, genus or other taxonomic See also:

CATEGORY (Gr. Karrlyopia, " accusation ")

category may be defined by polyp-characters or medusa-characters or by both combined . If our knowledge of the life-histories of these organisms. were perfect, their polymorphism would present no difficulties to classification; but unfortunately this is far from being the See also:

case . In the See also:

MAJORITY (Fr. majorite; Med. Lat. majoritas; Lat. major, greater)

majority of cases we do not know the polyp corresponding to a given medusa, or the medusa that arises from a given polyp.' Even when a medusa is seen to be budded from a polyp under observation in an See also:

AQUARIUM (plural aquaria)

aquarium; the difficulty is not always solved, since the freshly-liberated, immature medusa may differ greatly from the full-grown, sexually-mature medusa after several months of life on the high seas (see See also:

FIGS

figs. u, B,C, and 59, a, b, c) . To establish the exact relationship it is necessary not only to breed but to See also:

REAR

rear the medusa, which cannot always be done in 1 In some cases hydroids have been reared in aquaria from ova of medusae, but' these hydroids have not yet been found in the sea (See also:

Browne [to al) . (9) (1o) (12) (13), (14) (15) (t6) confinement . The alternative is to See also:

fish all stages of the medusa in its growth in the open sea, a slow and laborious method in which the See also:

CHANCE (through the O. Fr. cheance, from the Late Lat. cadentia, things happening, from cadere, to fall out, happen; cf. " case ")

chance of See also:

ERROR (Lat. error, from errare, to wander, to err)

error is very great, unless the See also:

SERIES (a Latin word from serere, to join)

series of stages is very See also:

COMPLETE

complete; At present, therefore, classifications of the Hydromedusae have a more or less tentative See also:

CHARACTER (Gr. xapareri7p, from xap&crew, to scratch)

character, and are liable to revision with increased knowledge of the life-histories of these organisms . Many See also:

groups See also:

bear at present two names, the one representing the group as defined by polyp-characters, the other as defined by medusa-characters . It is not even possible in all cases to be certain that the polyp-group corresponds exactly to the medusa-group, especially in See also:

minor systematic categories, such as families . The following is the See also:

main outline of the classification that is adopted in the present article .

Groups founded on polyp-characters are printed in See also:

ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

ordinary type, those founded on medusa-characters in italics . For See also:

DEFINITIONS

definitions of the groups see below . Sub-class Hydromedusae (Hydrozoa Craspedota) . See also:

Order I . Eleutheroblastea . , II . Hydroidea (Leptolinae) . Sub-order I . Gymnoblastea (Anthomedusae) . 2 . Calyptoblastea (Leptomedusae) . Order III .

Hydrocorallinae . „ IV . Graptolitoidea . V . Trachylinae . Sub-order r . Traehomedusae . 2 . Narcomedusae . Order VI . Siphonophora . Sub-order I .

Chondrophorida . 2 . Calycophorida . 3 . Physophorida . 4 . Cystophorida . Organization and See also:

MORPHOLOGY, (Gr. yopdsil, form)

Morphology of the Hydromedusae . As already stated, there occur in the Hydromedusae two distinct types of person, the polyp and the medusa; and either of them is capable of non-sexual See also:

REPRODUCTION

reproduction by budding, a See also:

PROCESS

process which may See also:

lead to the formation of colonies, composed of more or fewer individuals combined and connected together . The morphology of the group thus falls naturally into four sections—(r) the hydropolyp, (2) the polyp-See also:

COLONY (Lat. colonia, from colonus, a cultivator)

colony, (3) the hydro-medusa, (4) the medusa-colonies . Since, however, medusa-colonies occur only in one group, the Siphonophora, and divergent- views are held with regard to the morphological See also:

INTERPRETATION (from Lat. interpretari, to expound, explain, inter pres, an agent, go-between, interpreter; inter, between, and the root pret-, possibly connected with that seen either in Greek 4 p4'ew, to speak, or irpa-rrecv, to do)

interpretation of the members of a siphonophore, only the first three of the above sub-divisions of hydromedusa morphology will be dealt with here in a See also:

general way, and the morphology of the Siphonophora will be considered under the heading of the group itself . r .

The Hydropolyp (fig . I)—The general characters of this organism are described above and in the articles HYDROZOA and See also:

Pony's . It is rarely See also:

FREE

free, but usually fixed and incapable of locomotion . The See also:

FOOT

foot by which it is attached often sends out See also:

root-like processes—the hydro- rhiza (c) . The See also:

COLUMN (Lat. columna)

column (b) is generally See also:

long, slender and stalk- like (hydrocaulus) . Just below the See also:

CROWN

crown of tentacles, however, the See also:

BODY

body widens out to form a " See also:

head,” termed the hydranth (a), containing a See also:

STOMACH (Gr. vrbsaxos from arbga, a mouth)

stomach-like See also:

DILATATION (from Lat. dis-, distributive, and lotus, wide)

dilatation of the digestive cavity . On the upper See also:

FACE (from Lat. fades, derived either from facere, to make, or from a root fa-, meaning " appear "; cf. Gr. cbatvstv)

face of the hydranth the crown of tentacles (t) surrounds the peristome, from which rises the conical hypostome, bearing the mouth at its extremity . The general ectoderm covering the See also:

SURFACE

surface of the body has entirely lost the cilia present in the earlier larval stages (planula), and may be naked, or clothed in a cuticle or exo- See also:

SKELETON

skeleton, the perisarc (ps), which in its simplest See also:

CONDITION (Lat. condicio, from condicere, to agree upon, arrange; not connected with conditio, from condere, conditum, to put together)

condition is a chitinous membrane secreted by the ectoderm . The perisarc when present invests the hydrorhiza and hydrocaulus; it may stop short below the hydranth, or it may extend farther . In general there are two types of exoskeleton, characteristic of the two See also:

PRINCIPAL

principal divisions of the Hydroidea . In the Gymnoblastea the perisarc either stops below the hydranth, or, if continued on to it, forms a closely-fitting investment extending as a thin cuticle as far as the bases of the tentacles (e.g . Bimeria, see G .

J . See also:

ALLMAN, GEORGE JAMES (1812-1898)

Allman [ii,i pl. xii. figs. r and 3) . In the Calyptoblastea the pensarc is always continued above the From Allman's Gymn oblsstic Hydroids, by permiccion of the See also:

COUNCIL (Lat. concilium, from cum, together, and the root cal, to call)

Council of the See also:

Ray Society . hydrocaulus, and forms a See also:

CUP (in O.E. cuppe; generally taken to be from Late Lat. cuppa, a variant of Lat. cupa, a cask, cf. Gr. KuaeXXov)

cup, the hydrangium or hydrotheca (h, t), See also:

STANDING

standing off from the body, into which the hydranth can be retracted for shelter and See also:

PROTECTION

protection . The See also:

ARCHITECTURE (Lat. architectura, from the Gr. ap)(LTEKrwv, a master-builder)

architecture of the hydropolyp, simple though it be, furnishes a long series of See also:

variations affecting each See also:

PART

part of the body . The greatest variation, however, is seen in the tentacles . As regards number, we find in the aberrant forms Protohydra and Microhydra tentacles entirely absent . In the curious hydroid Monobrachium tentacle is present, and the same is the case in Clathrozoon; in Amphibrachium and in See also:

Lar (fig . II, A) the polyp bears two tentacles only . The reduction of the tentacles in all these forms may be correlated with their mode of life, and especially with living in a See also:

CONSTANT, BENJAMIN (1845-1902)

constant current of water, which brings See also:

food-particles always from one direction and renders a complete whorl or circle of tentacles unnecessary . Thus Microhydra lives amongst Bryozoa, and appears to utilize the currents produced by these animals . Protohydra occurs in See also:

OYSTER

oyster-See also:

banks and Monobrachium also grows on the shells of bivalves, and both these hydroids probably fish in the currents produced by the lamellibranchs .

Amphibrachium grows in the tissues of a sponge, Euplectella, and protrudes its hydranth into the See also:

canal-system of the sponge; and Lar grows on the tubes of the See also:

WORM

worm Sabella . With the exception of these forms, reduced for the most part in correlation with a semi-parasitic mode of life, the tentacles are usually numerous . FIG . 3.—See also:

DIAGRAM

Diagram of It is rare to find in the polyp a See also:

REGULAR

regular, Corymor phis . A, A hydrisymmetrical disposition of the tentacles form person giving rise as in the medusa . The See also:

PRIMITIVE

primitive number to medusiform persons of four in a whorl is seen, however, in by budding from the Stauridium (fig . 2 ? ' and Cladonema margin of the disk; B, (Allman [I], pl. xvii.), and in Clavatella free See also:

SWIMMING (from " swim," A.S. swimman, the root being common in Teutonic languages)

swimming medusa each whorl consists regularly of - eight See also:

Forbes) (Allman, loc. cit. pl. xviii.) . As a See also:

RULE

rule, detached ro of the same, however, the number in a whorl is deached from the same, irregular . The tentacles may form a twith omedus e) and dionlyy lly single whorl, or more than one; thus in Corymorpha (fig . 3) and Tubular= one ne tentacle. and (After All-(fig . 4) there are two circlets; in Staur- See also:

man) .

idsum (fig . 2) several; in Coryne and Cordylophora the tentacles are scattered irregularly over the elongated hydranth . As regards form, the tentacles show a number of types, of which the most important are (I) filiform, i.e. cylindrical or tapering from 'The See also:

numbers in square brackets [ ] refer to the bibliography at the end of this article; but when the number is preceded by the word Hydrozoa, it refers to the bibliography at the end of the article HYDROZOA . „ „ a, Hydranth; b, Hydrocaulus; c, Hydrorhiza; t, Tentacle; ps, Perisarc, forming in the region of the hydranth a cup or hydrotheca(h, t), —which, however,is only found in polyps of the order Calyptoblastea . a single See also:

BASE

base to extremity, as in Clava (fig . 5); (2) capitate, i.e. knobbed at the extremity, as in Coryne (see Allman, loc. cit. p1. iv.); (3) branched, a rare form in the polyp, but seen in Cladocoryne (see Allman, loc. cit. p . 38o, fig . 82) . Sometimes more than one type of form is found in the same polyp; in Pennaria and Stauridium (fig . 2) the upper whorls are capitate, the See also:

LOWER

lower filiform . Finally, as regards structure, the tentacles may retain their primitive hollow nature, or become solid by obliteration of the axial cavity . The hypostome of the hydropolyp may be small, or, on the other See also:

hand, as in Eudendrium (Allman, loc. cit. pis. xiii., xiv.), large and See also:

trumpet - shaped .

In the curious polyp Myriothela the body of the polyp is differentiated into nutritive and reproductive portions . See also:

Histology . — The ectoderm of the hydropolyp is chiefly sensory, contractile and protective in See also:

FUNCTION

function . It may also be glandular in places . It consists of two regions, an See also:

EXTERNAL

external See also:

EPITHELIAL, ENDOTHELIAL

epithelial layer. and a more See also:

INTERNAL

internal sub-epithelial layer . The epithelial layer consists of (I) so-called " indifferent " cells secreting the perisare or cuticle and modified to form glandular cells in places; for example, the adhesive cells in the foot . (2) Sensory cells, which may be fairly numerous in places, especially on the tentacles, but which occur always scattered and isolated, never aggregated to form sense-organs as in the medusa . (3) Contractile From Allman's Gynasobiastic Hydroids, by permission of the Council of the Ray Society . or myo-epithelial cells, with the See also:

CELL (from Lat. cella, probably from an Indo-European kal —seen in Lat. celare, to hide; another suggestion connects the word with Lat. cera, wax, taking the original meaning to refer to the honeycomb)

cell prolonged at the base into a contractile muscle-fibre (fig . 6, B) . In the hydropolyp the ectodermal muscle-See also:

fibres are always directed longitudinally . Belonging primarily to the epithelial layer, the See also:

MUSCULAR

muscular cells may become secondarily sub-epithelial .

The sub-epithelial layer consists primarily of the so-called interstitial cells, lodged between the narrowed basal portions of the epithelial cells . From them are developed two distinct types of histological elements; the genital cells and the cnidoblasts or See also:

MOTHER

mother-cells of the nematocysts . The sub-epithelial layer thus primarily constituted may be recruited by See also:

IMMIGRATION (from Lat. in, into, and tnigrare, to depart)

immigration from without of other elements, more especially by nervous (ganglion) cells and rntisclecells derived from the epithelial layer . In its fullest development, therefore, the sub-epithelial layer consists of four classes of cellelaments . The genital cells are simple wandering cells (archaeocytes), at first See also:

minute and without any specially distinctive features, until they begin to develop into germ-cells . According to Wulfert [6o] the primitive germ-cells of Gonoihyraea can be distinguished soon after the fixation of the planula, appearing amongst the interstitial cells of the ectoderm . The germ-cells are capable of extensive migrations, not only in the body of the same polyp, but also from See also:

PARENT, SIMON NAPOLEON (1855– )

parent to bud through many non-sexual generations of polyps in a colony (A . See also:

WEISMANN, AUGUST (1834– )

Weismann [58]) . The cnidoblasts are the mother-cells of the nematocysts, each' cell producing one nematocyst in its interior . The complete nematocyst (fig . 7) is a spherical or See also:

OVAL (Lat. ovum, egg)

oval See also:

CAPSULE (from the Lat. capsula, a small box)

capsule containing a hollow See also:

THREAD (0. Eng. praed, literally, that which is twisted, prawan, to twist, to throw, cf. " throwster," a silk-winder, Ger. drehen, to twist, turn, Du. draad, Ger. Draht, thread, wire)

thread, usually barbed, coiled in its interior . The capsule has a See also:

double See also:

wall, an See also:

OUTER

outer one (o.c.), tough and rigid in nature, and an inner one (i.c.) of more flexible consistence .

The outer wall of the capsule is in-complete at one See also:

pole, leaving an See also:

APERTURE (from Lat. aperire, to open)

aperture through which the thread is discharged . The inner membrane is continuous with the wall of the hollow thread at a spot immediately below the aperture in the outer wall, so that the thread itself (f) is simply a hollow prolongation of the wall of the inner capsule inverted and pushed into its cavity . The entire nematocyst is enclosed in the cnidoblast which form d it . When the nematocyst is completely developed, the cnidobl4st passes outwards so as to occupy a superficial position in the ectoderm, and a delicate protoplasmic process of sensory nature, termed the cnidocil (cn) projects from the cnidoblast like a See also:

FINE

fine See also:

HAIR (a word common to Teutonic languages)

hair or ciliufn . Many points in the development and mechanism of the nematocyst are disputed, but it is tolerably certain (i) that the cnidocil is of sensory nature, and that stimulation, by contact with See also:

PREY (0. Fr. preie, mod. proie, Lat. praeda, booty, from prae and the root hed—seen in prehendere, prendere, to grasp)

prey or in other ways, causes a reflex See also:

DISCHARGE (adapted from the O. Fr. discharge, modern decharge, from a med. Lat. discargare, to unload, dis- and earn care, to load, cf. " charge ")

discharge of the nematocyst; (2) that the discharge is an explosive See also:

CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change whereby the in-turned thread is suddenly everted and turned inside out, being thus shot through the opening in the outer wall of the capsule, and forced violently Into the tissues of the prey, or, it may be, of an enemy; (3) that the thread inflicts not merely a See also:

MECHANICAL

mechanical See also:

WOUND (O. Eng. wund,connected with a Teutonic verb, meaning to strive, fight, suffer, seen in O. Eng. winnan, whence Eng. " win ")

wound, but instils an irritant See also:

POISON

poison, numbing and paralysing in its See also:

ACTION

action . The points most in dispute are, first, how the explosive discharge is brought about, whether by pressure exerted external to the capsule (i.e. by contraction of the cnidoblast) or by internal pressure . N . Iwanzov [271 has brought forward strong grounds for the latter view, pointing out that the cnidoblast has no contractile mechanism and that measurements show discharged capsules to be on the See also:

AVERAGE

average slightly larger than undischarged ones . He believes that the capsule contains a sub-stance which swells very rapidly when brought into contact with water, and that in the undischarged condition the capsule has its opening closed by a plug of See also:

PROTOPLASM

protoplasm (x, fig . 7) which prevents rh- See also:

ACCESS (Lat. accessus)

access of water to the contents; when the enidocil is'stimulated it sets in action a mechanism or perhaps a series of chemical changes by which the plug is dissolved or removed; as a result water penetrates into the capsule and causes its contents to swell, with the result that the thread is everted violently . A second point of dispute concerns the spot at which the poison is lodged . Iwanzov believes it to be contained within the -thread itself before discharge, and to. be introduced into the tissues of the prey by' the eversion of the thread .

A third point of dispute is whether the nematocysts are formed in situ, or whether the cnidoblattts migrate with them to the region where they' are most needed; the fact that in Hydra, for example, there are no interstitial cells in the tentacles, where nematocysts are very abundant, is certainly in favour of the view. that the cnidoblasts migrate on to the tentacles from the body, and that like the genital cells the cnidoblasts are wandering cells . The muscular See also:

TISSUE (Fr. tissu, tissue, participle of tisser, Lat. texere, to weave)

tissue consists primarily of processesfrom the bases of the epithelial cells, processes which are contractile in nature and may be distinctly striated . A a, Undischarged nematocyst . b, Commencing discharge . c, Discharge complete. forming a sub-epithelial cn, Cnidocil. contractile layer, de_ N, See also:

NUCLEUS (Lat. for the kernal of a nut, nux, the stone of fruit)

Nucleus of cnidoblast. veloped chiefly tn the ten- o.c, Outer capsule. tacles of the polyp . The x, Plug closing the opening of the See also:

EVOLUTION

evolution of the ganglion- outer capsule. cells, is probably similar; i.c., Inner capsule, continuous with the an epithelial cell develops wall of the filament, f. processes of nervous nature b, Barbs. from the base; which come into connexion with the bases of the sensory cells, with the muscular cells, and with the similar processes of other See also:

NERVE (Lat. nervus, Gr. vevpov, a bowstring)

nerve-cells; next the nerve-cell loses its connexion with the outer epithelium and becomes a sub-epithelial ganglion-cell which is closely connected with the muscular layer, conveying stimuli from the sensory cells to the contractile elements . The ganglion-cells of Hydromedusae are generally very small . In the polyp the nervous tissue is always in the form' of a scattered plexus, never See also:

con- centrated to form a definite nervous system as in the medusa . The endoderm of the polyp is typically a flagellated epithelium of large cells (fig . 6), from the bases of which arise contractile muscular processes lying in the See also:

PLANE

plane of the transverse See also:

section of the body . In different parts of the coelenteron the endoderm' may be of three principal types-(I) digestive endoderm, the primi- tive type, with cells of large See also:

SIZE

size and considerably vacuolated, found in the hydranth; sortie of these cells may become See also:

SPECIAL

special glandular cells, without flagella' or contractile processes; (2) circulatory endoderm, without vacuoles and without basal contractile processes, found in the hydrorhiza and hydrocaulus; (3) supporting endoderm (fig . 8), seen in solid tentacles as a See also:

ROW, JOHN (c. 1525—1580)

row of cubical vacuolated cells, occupying the See also:

AXIS (Lat. for " axle ")

axis of the tentacle, greatly resembling notochordal tissue, particularly that of Arnphioxus at a certain stage of development; as a See also:

FOURTH

fourth variety of endodermal cells excretory cells should perhaps be reckoned, as seen in the pores in the foot of Hydra and elsewhere (cf .

C . Chun, HYDROZOA PP . 314, 315)- The mesogioea in the hydropolyp is a thin elastic layer, in whichmaybe lodged the muscular fibres and ganglion cells mentioned above. but which never contains any connective tissue or skeletogenous cells or any other See also:

KIND (O. E. ge-cynde, from the same root as is seen in " kin," supra)

kind of special mesogloeal corpuscles . 2 . The Polyp-colony.---All known hydropolyps possess the See also:

POWER [WILLIAM GRATTAN] TYRONE (1797-1841)

power of reproduction by budding, and the buds produced may become either polyps or medusae . The buds may all be-come detached after a See also:

time and give rise to See also:

SEPARATE

separate and in-dependent individuals, as in the common Hydra, in which only polyp-individuals are produced and sexual elements are developed upon the polyps themselves; or, on the other hand, the polyp individuals produced by budding may remain permanently in connexion with the parent polyp, in which case sexual elements are never developed on polyp-individuals but only on medusa-individuals, and a true colony is formed . Thus the typical hydroid colony starts from a " founder polyp, which in the vast majority of cases is fixed, but which may be floating, as in Nemopsis, Pelagohydra, &c . The founder-polyp usually produces by budditlg polyp-individuals, and these in th,pir turn produce other buds . The polyps are all non-sexual individuals whose function is purely nutritive . After a time the polyps, or certain of them, produce by budding medusa-individuals, which sooner or later develop sexual elements; in some cases, however, the founder.. polyp remains solitary, that is to say, does not produce polyp-buds, but only medusa-buds, from the first (Corymorpha, fig . 3, Myriothela, &c.) . In primitive forms the medusa-individuals are set free before reaching sexual maturity and do not con-See also:

TRIBUTE (Lat. tributurn, a stated payment, contribution)

tribute anything to the colony .

In other cases, however, the medusa-individuals become sexually mature while still attached to the parent polyp, and are then not set free at all, but become appanages of the hydroid colony and undergo degenerative changes leading to reduction and even to complete obliteration of their See also:

ORIGINAL

original medusan structure . I n this way the hydroid colony becomes composed of two portions of different function, the nutritive " trophosome," composed of non-sexual polyps, and the reproductive "gonosome," composed of sexual medusa-individuals, which never exercise a nutritive function while attached to the colony . As a general rule polyp-buds are produced from the hydrorhiza and hydrocaulus, while medusa-buds are formed on the hydranth . In some cases, however, medusa= buds are formed on the hydrorhiza, as in Hydrocorallines . In such a colony of connected individuals, the exact limits of the separate " persons " are not always clearly marked out . Hence it is necessary to distinguish between,first,the "zooids," indicated in the case of the polyps by the hydranths, each with mouth andtentacles; and, secondly, the "coenosarc, or common flesh, which cannot be assigned more to one individual than another, but consists of a more or less complicated network of tubes, corresponding to the hydrocaulus and hydrorhiza of the primitive See also:

INDEPENDENT

independent polypp--individuaI. fihe coenosarc constitutes a system by which the digestive Cavity of any one polyp is put into communication with that of any other individual either of the trophosome or gonosdme . In this manner the food absorbed by one individual contributes to the welfare of the whole ' colony, and the coenosarc has the the epithelium and come to See also:

lie entirely beneath it, From See also:

GEGENBAUR, CARL (1826-1903)

Gegenbaur's Elements of Cost-}arative See also:

Anatomy . Fia . 8.— Vacuolated Endoderm Cells of cartilaginous consistence from the axis of the tentacle of a Medusa (Canino) . From Allman's Gymnoblastic Hydroids, by permission of the Council of the Ray Society . ' From Allman's Gymnoblastie by permission of the Council of Society . Hydroids, the Ray theoretically, of unlimited growth in a See also:

VERTICAL (from Lat. vertex, highest point)

vertical direction, and as it grows up it throws out buds tight and See also:

LEFT

left alternately, so that the first bud produced by it is the lowest down, t he second bud is above the first, the third above this again, and so on .

Each bud produced function of circulating and distributing nutriment through the colony . The hydroid colony shows many variations in form and architecture which depend simply upon differences in the methods in which polyps are budded . In the first See also:

PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place, buds may be produced only from the hydrorhiza, which grows out and branches to form a basal stolon, typically See also:

net-like, spreading over the substratum to which the founder-polyp attached itself . From the stolon the daughter-polyps grow up vertically . The result is a spreading or creeping colony, with the coenosarc in the form of a root-like See also:

HORIZONTAL

horizontal network (fig . 5, B; II, A) . Such a colony may undergo two principal modifications . The meshes of the basal network may become very small or virtually obliterated, so that the coenosarc be-comes a crust of tubes tendingtofusetogether, and covered over by a common perisarc . Encrusting colonies of this kind are seen in Clava squamata (fig . 5, A) and Hydractitria (figs . 9, 1o), the latter having the perisarc A calcified . A further very important modifi- After See also:

Hincks, Forbes, and Browne .

A and B modified cation is seen when the from Hincks; C modified from Forbes's Brit . Naked-eyed Med tubes of the basal urae . perisarc do not remain of its Medusa, Willia stellata . A, colony of but grow in all planes See also:

Las; B and C, See also:

young and adult medusae. forming a See also:

FELT (cognate with Ger. Filz, Du. vilt, Swed. and Dan. fill; the root is unknown; the word has given Med. Lat. filtrum, " filter ")

felt-See also:

WORK

work; the result is a massive colony, such as is seen in the so-called Hydrocorallines (fig . 6o), where the interspaces between the coenosatml tubes are filled tip with calcareous See also:

MATTER

matter, or caenosteum, replacing the chitinous perisarc . The result is a stony, solid See also:

mass, which contributes to the See also:

BUILDING

building up of See also:

CORAL

coral reefs . In massive colonies of this kind no See also:

sharp distinction can be See also:

DRAWN

drawn between hydrorhiza and hydro- Y.caulus in the ~ooeeosarc; it is practically all hydrorhiza . Massive colonies may assume es- various forms and are often branching or See also:

tree-like . A See also:

FUR (connected with O. Fr. forre, a sheath or case; so " an outer covering ")

fur- - they peculiarity of this type of colony is that the entire coeno- sarcal complex is covered externally by a common layer of ectoderm ; it is not clear how this cohering layer is developed . In the second place, the buds may be produced from the hydrocaulus, growing out laterally from it; the result is an arborescent, tree-like colony (figs . 12, 13) . Budding from the hydrocaulus may be combined with budding from the hydrorhiza, so that numerous branching colonies arise from a common basal stolon .

In the formation of arborescent colonies, two sharply distinct types of budding are found, which are best deseribed in botanical terminology as the monopodial or racemose, and the sympodial or cymose types respectivell, ; each is characteristic of one of the two sub-orders of the Hydrrndea, the Gynlnoblastea and Calyptoblastea . In the monopodial method (figs.' i2, 14) the founder-polyp is, by the founder proceeds to grow and to bud in the. same way as the founder did, producing a See also:

side See also:

BRANCH (from the Fr. branche, late Lat. branca, an animal's paw)

branch of the main See also:

STEM (O. Eng. staefn, stemn, cf. Du. stain, Ger. Stamm, &c., probably related to " staff ")

stem . Hence, in a colony of gymnoblastic hydroids, the See also:

OLDEST

oldest polyp of each system, that is to say, of the main stem or of a branch, is the topmost polyp; the youngest polyp of the system is the one nearest to the topmost polyp; and the axis of the system is a true axis . In the sympodial method of budding, on the other hand, the founder-polyp is of limited growth, and forms a bud from its side, which is also of limited growth, and forms a bud in its turn, and so on (figs. i5, 16) . Hence, in a colony of calyptoblastic hydroids, the oldest polyp of a system is the lowest; the youngest polyp is the See also:

TOP (cf. Dan. top, Ger. Topf, also meaning pot)

top- most one; and the axis of the system is a false axis composed of portions of each of the consecutive polyps . In this method of budding there are two types.' In one, the biserial type (fig . 15) ,the polyps See also:

pro- duce buds right -b3 and left alter- nately, so that the hydranths are arranged in a zig- zag See also:

FASHION (adapted from Fr. facon, agon, Lat. factio, making, facere, to do or make)

fashion, form- See also:

ing a " scorpioid cyrim," as in Obelia and Serlularia . In the other, the uni- serial type (fig.'6), the buds are formed always on 3 1 5 the same side, forming a " heli- serial arrangement becomes masked later by secondary torsions of the hydranths . In a colony formed by sympodial budding, a polyp always produces first a bud, which contributes to the system to which it belongs, i.e. continues the stem or branch of which its parent forms a part . The F polyp may then form a second d bud, which becomes the starting point of a new system, the j t beginning, that is, of a new 4`\ branch; and even a third bud, I starting yet another system, may be produced from the same polyp . Hence the colonies of Calyptoblastea may be coinplexly branched, and the bud-ding may be biserial through-out, uniserial ,throughout, or partly one, partly the other . Z b 4 Thus in Plumularidae (figs .

17, biserial sbudding; eachnpolyp budding, uniserial type, shown on the main stem forms a in four stages (1-4) . F, founder- second' bud, which usually polyp; I, 2, 3, See also:

SUCCESSION (Lat. successio, from succedere, to follow after)

succession of polyps forms a side branch or pinnule budded from the founder. by uniserial budding . In this way are formed the familiar feathery colonies of Plumularia, in which the pinnules are. all in one plane, while in the allied Anlennularia the pinnules are arranged in whorls See also:

ROUND (O. Fr. rond, Lat. rotundus, the Fr. is the source also of Du. rond; Ger., Swed., Dan. and Nor. rond)

round the main biserial stem . The pinnules never branch again, since in the uniserial mode of budding a polyp never forms a second polyp-bud . On the other hand, a polyp on the main stem may form a second bud which, instead of forming a pinnule by uniserial budding, produces by biserial bud-ding a branch, from which pinnules arise as from the main stem (fig . 18—3, 6) . Or a polyp on the main stem, after having budded a second time to form a pinnule, may give rise to a third bud, which starts a new biserial in an interesting manner by H . Driesch [131, to whose See also:

MEMOIRS

memoirs the reader must be referred for further details . Individualization of Polyp-Colonies.—As in other cases where See also:

animal colonies are formed by organic See also:

union of separate individuals, there is ever a tendency for the polyp-colony as a whole to See also:

ACT (Lat. actus, actum)

act as asingle individual, and for the members to become subordinated to the needs of the colony and to undergo specialization for particular functions, with the result that they simulate organs and their individuality becomes masked to a greater or less degree . Perhaps the earliest of such specializations is connected with the reproductive function . Whereas primitively any polyp in a colony may produce medusa-buds, in many hydroid colonies medusae are budded only by certain polyps termed blastostyles (fig. to, b) . At first not differing in any way from other polyps (fig .

5), the blastostyles gradually lose their nutritive function and the organs connected with it; the mouth and tentacles disappear, and the blastostyle obtains the nutriment necessary for its activity by way of the coenosarc . In the Calyptoblastea, where the polyps are protected by special capsules of the perisarc, the gonothecae en-closing the blastostyles differ from the hydro-thecae protecting the hydranths (fig . 54) . In other colonies the two functions of the nutritive polyp, namely, See also:

CAPTURE (from Lat. capere, to take; Fr. prise maritime; Ger. Wegnahme)

capture and digestion of food, may be shared between different polyps (fig . 1o) . One class FIG . 18.—Diagram showing method of polyps, the dactylozoids of branching in the Plumularia-type; (dz), lose their mouth and compare with fig . 17 . Polyps 3 and 6, stomach, and become elon- instead of producing uniserial pinnules, gated and tentacle-like, have produced biserial branches (3', 32, showing great activity of 3', 34; 61-63), which give off uniserial See also:

MOVEMENT

movement . Another class, branches in their turn . the gastrozoids (gz), have the tentacles reduced or absent, but have the mouth and stomach enlarged . The dactylozoids capture food and pass it on to the gastrozoids, which See also:

SWALLOW (A. S. swalewe, Icel. svala, Du. zwaluw, Ger. Schwalbe)

swallow and See also:

DIGEST

digest it .

Besides the three types of individual above mentioned, there are other appendages of hydroid colonies, of which the individuality is doubtful . Such are the " guard-polyps " (machopolyps) of Plumularidae, which are often regarded as individuals of the nature of dactylozoids, but from a study of the mode of budding in this hydroid See also:

FAMILY

family Driesch concluded that the guard-polyps were not true polyp-individuals, although each is enclosed in a small protecting cup of the perisarc, known as a nematophore . Again, the spines arising from the basal crust of Podocoryne have been interpreted by some authors as reduced polyps . 3 . The Medusa . —In the Hydromedusae the medusa-individual occurs, as already stated, in one of two conditions, either as an independent organism leading a true life in the open seas, or as a subordinate individuality in the hydroid colony, from which it is never set free; it then becomes a See also:

mere reproductive appendage or gono- phore, losing See also:

SUE, EUGENE [JosEPH MARIE] (1804—1857)

sue FIG . 19.—Diagram showing method of branchcessively its organs ing in the Aglaophenia-type . Polyp 7 has pro-of sense, loco- duced as its first bud, 8; as its second bud, al, See also:

motion and nutri- which starts a uniserial pinnule; and as a third tion, until its bud I', which starts a biserial branch (IP-VP) medusoid nature that repeats the structure of the main stem and and organization gives off pinnules . The main stem is indicated become scarcely by –•–•-•, the new stem by • • • • • . recognizable . Hence. it is convenient to consider the morphology of the medusa from these two aspects . (a) The Medusa as an Independent Organism.--The general structure and characteristics of the medusa are described elsewhere (see articles HYDROZOA and MEDUSA), and it is only necessary here to See also:

DEAL

deal with the peculiarities of the Hydromedusa .

As regards See also:

HABIT (through the French from Lat. habitus, from habere, to have, hold, or, in a reflective sense, to be in a certain condition; in many of the English senses the French use habitude, not habit)

habit of life the vast majority of Hydromedusae are tt pelagic organisms, floating on the surface of the open sea, propelling I themselves feebly by the pumping movements of the umbrella produced by contraction of the sub-umbral musculature, and capturing their prey with their tentacles . The genera Cladonema (fig . 20) and Clava- tella (fig . 21), how- ever,are See also:

AMBULATORY (Med. Lat. ainbulatorium, a place for walking, from ambulare, to walk)

ambulatory, creeping forms,living in See also:

rock-pools and walking, as it were, on the tips of the proximal branches of each of the tentacles, while the remaining branches serve for capture of food . Cladonema still has the typical medusan structure, and is able to swim about, but in Clavatella the um- brella is so much re- duced that swimming is no longer possible . The remarkable medusa Mnestra parasites isecto-See also:

para- sitic throughout life on the pelagic mollusc Phyllirrhoe, attached to it by the sub- umbral surface, and its tentacles have become rudimentary or absent . It is inter- esting to See also:

note that Mnestra has been shown by J . W . Fewkes [15] and R . T . See also:

GUNTHER, JOHANN CHRISTIAN (1695-1723)

Gunther [191 to belong to the same family (Cladone- midae) as Cladonema and Clavatella, and it is reasonable to suppose that the non-parasitic ancestor of Mnestra was, like the other two genera, an ambulatory medusa which acquired See also:

LOUSE (O. Eng. leis, cf. Du. luis, Ger. Laus, Dan. and Swed. lus)

louse-like habits . In some ,.t species of the genus Cunina(Narcomedusae) the youngest individuals (actinulae) are parasitic on other medusae (see below), but in later life the parasitic habit is abandoned .

No other instances are known of sessile habit in Hydromedusae . The external form of the Hydromedusae varies from that of a deep See also:

bell or See also:

THIMBLE

thimble, characteristic of the Anthomedusae, to the shallow saucer-like form characteristic of the Leptomedusae . It is usual for the umbrella to have an even, circular, uninterrupted margin; but in the order Narcomedusae secondary down-growths between the tentacles produce a lobed, indented margin to the umbrella . The marginal tentacles are rarely absent in non-parasitic forms, and are typically four in number, cor- responding to the four perradii marked by the radial canals . Interradial ten- tacles may be also developed, so that the See also:

TOTAL

total number present may be in- creased to eight or to an indefinitely large number . In Willia, Geryonia, &c., however, the tentacles and radial canals are on the See also:

plan of six instead of four (figs. it and 26) . On the other hand, in some cases the tentacles are less in number than the perradii; in Corymorpha (figs . 3 and 22) there is but a single tentacle, while two are found in Ampphinema and Gemmaria (An- thomedusae), and in Solmundella bitentaculata (fig . 67) and Aeginopsis hensenii (fig . 23) (Narcomedusae) . The tentacles also vary considerably in other ways than in number: first, in form, being usually simple, with a basal bulb, but in Cladonem- idae they are branched, often in complicated fashion; secondly, in grouping, being usually given off singly, and at regular intervals from the margin of the umbrella, but in Margelidae and in some Trachomedusae they are given off in tufts or bunches (fig . 24); thirdly. in position and origin, being usually implanted on the extreme edge of the umbrella, but in Narcomedusae they become secondarily shifted and are given off high up on the ex-umbrella (figs .

23 and 25) ; and, fourthly, in structure, being hollow or solid, as in the polyp . In some medusae, for instance, the remarkable deep-sea family Pectyllidae, the tentacles may bear suckers, by which the animal may attach itself temporarily . It should be mentioned t finally that the tentacles are very contractile and extensible, and may therefore present themselves, in one and the same Individual, as long, drawn-out threads, or in the form of short corkscrew - like ringlets; they may stream downwards from the sub-umbrella, or be held out horizontally, or be directed upwards over the ex-umbrella (fig . 23) . Each species of After O, See also:

MAAS, JOSEPH (1847-1886)

Maas, See also:

Die eraspedoten Medusen der See also:

PLANKTON

Plankton Expedition, by permission of See also:

Lipsius and Tischer . medusa usually has a characteristic method of carrying its tentacles . The sub-umbrella invariably shows a velum as an inwardly projecting See also:

ridge or rim at its margin, within the circle of tentacles; hence the medusae of this sub-class are termed craspedote . The manubrium is absent altogether in the fresh-water medusa Limnocnida, in which the See also:

DIAMETER (from the Cr. &a, through, µErpov, measure)

diameter of the mouth exceeds See also:

HALF

half that of the umbrella; on the other hand, the manubrium may attain a great length, owing to the centre of the sub-umbrella with the stomach being drawn into it, as it were, to form a long See also:

PROBOSCIS

proboscis, as in Geryonia . The mouth may be a simple, circular See also:

PORE

pore at the extremity of the manubrium, or by folding of the edges it may become square or shaped like a Maltese See also:

CROSS

cross, with four corners and four lips . The corners of the mouth may then be drawn out into lobes or lappets, which may have a branched or fringed outline (fig . 27), and in Margelidae the subdivisions of the fringe simulate tentacles (fig . 24) .

The internal anatomy of the Hydromedusae shows numerous variations . The stomach may be altogether lodged in the manubrium, from which the radial canals then take origin directly as in Geryonia (Trachomedusae) ; it may be with or without gastric pouches . The radial canals may be simple or branched, primarily four, rarely six in number . The See also:

RING (O.E. hring; a word common to Teutonic languages; and probably cognate with the Lat. circus, Gr. KtpKOS or KpLKOS, Skt'. chakra, wheel, circle, cf. also " harangue "); in art, a band of circular shape of varying sizes, made of any material and used f

ring-canal is drawn out in Narcomedusae into festoons corresponding with the lobes of the margin, and may be obliterated altogether (See also:

SAMARIA

Samaria) . In this order the radial canals are represented only by wide gastric pouches, and in the family Solmaridae are suppressed altogether, so that the tentacles and the festoons of the ring-canal arise directly from the stomach . In Geryonia, centripetal canals, ending blindly, arise from the ring-canal and run in a radial direction towards the centre of the umbrella (fig . 26) . Histology of the Hydromedusa.—The histology described above for the polyp may be taken as the primitive type, from which that From Allman's Gymnoblastic Hydroids, by permission of the Council of the Ray Society . From Allman's Gymnoblastic Hydroids, by permission of the Council of the Ray Society . After E . T, Browne, from Prot . Zool .