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ORDER VI

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Originally appearing in Volume V14, Page 161 of the 1911 Encyclopedia Britannica.
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ORDER VI. Siphonophora.—Pelagic floating Hydrozoa with great differentiation of parts, each performing a special function; generally regarded as colonies showing differentiation of individuals in correspondence with a physiological division of labour. A typical Siphonophore is a stock or cormus consisting of a number of appendages placed in organic connexion with one another by means of a coenosarc. The coenosarc does not differ in structure from that already described in colonial Hydrozoa. It consists of a hollow tube, or tubes, of which the wall is made up of the two body-layers, ectoderm and endoderm, and the cavity is a continuation of the digestive cavities of the nutritive and other appendages, i.e. of the coelenteron. The coenosarc may consist of a single elongated tube or stolon, forming the stem or axis of the cormus on which, usually, the appendages are arranged in groups termed cormidia; or it may take the form of a compact mass of ramifying, anastomosing tubes, in which case the cormus as a whole has a compact form and cormidia are not distinguish-able. In the Disconectae the coenosarc forms a spongy mass, the "centradenia," which is partly hepatic in function, forming the so-called liver, and partly excretory. The appendages show various types of form and structure corresponding to different functions. The cormus is always differentiated into two parts; an upper portion termed the nectosome, in which the appendages are locomotor or hydrostatic in function, that is to say, serve for swimming or floating; and a lower portion termedbelow, but in enumerating the various types of appendages it is convenient to discuss their morphological interpretation at the same time. In the nectosome one or more of the following types of appendage occur: I. Swimming-bells, termed nectocalyces or nectophores (fig. 68, k), absent in Chondrophorida and Cystophorida; they are contractile and resemble, both in appearance, structure and function, the umbrella of a medusa, with radial canals, ring-canal and velum; but they are without manubrium, tentacles or sense-organs, and are always bilaterally symmetrical, a peculiarity of form related with the fact that they are attached on one side to the stem. A given cormus may bear one or several nectocalyces, and by their con-tractions they propel the colony slowly along, like so many medusae harnessed together. In cases where the cormus has no pneumatophore the top-most swimming bell may contain an oil-reservoir or oleocyst. 2. The pneumatophore or air-bladder (fig. 68, n), After A. Agassiz, from Lankester's Treatise on for passive locomotion, Zoology. forming a float which FIG. 69.—Porpita, seen from above, keeps the cormus at or showing the pneumatophore and ex-near the surface of the panded palpons. water. The pneumato- phore arises from the ectoderm as a pit or invagination, part of which forms a gas-secreting gland, while the rest gives rise to an air-sack lined by a chitinous cuticle. The orifice of invagination forms a pore which may be closed up or may form a protruding duct or funnel. As in the analogous swim-bladder of fishes, the gas in the pneumatophore can be secreted or absorbed, whereby the specific gravity of the body can be diminished or increased, so as to cause it to float nearer the surface or at a deeper level. Never more than one pneumatophore is found in a cormus, and when present it is always situated at the highest point above the swimming bells, if these are present also. In Velella the pneumatophore becomes of complex structure and sends air-tubes, lined by a chitin and resembling tracheae, down into the compact coenosarc, thus evidently serving a respiratory as well as a hydrostatic function. Divergent views have been held as to the morphological significance of the pneumatophore. E. Haeckel regarded the whole structure as a glandular ectodermal pit formed on the exumbral surface of a medusa-person. C. Chun and, more recently, R. Woltereck [89], on the other hand, have shown that the ectodermal pit which gives rise to the pneumatophore represents an entocodon. Hence the cavity of the air-sack is equivalent to a sub-umbral cavity in which no manubrium is formed, and the pore or orifice of invagination would represent the margin of the umbrella. In the wall of the sack is a double layer of endoderm, the space between which is a continuation of the coelenteron. By coalescence of the endoderm-layers, the coelenteron may be reduced to vessels, usually eight in number, opening into a ring-sinus surrounding the pore. Thus the disposition of the endoderm-cavities is roughly comparable to the gastrovascular system of a medusa. The difference between the theories of Haeckel and Chun is connected with a further divergence in the interpretation of the stem or axis of the cormus. Haeckel regards it as the equivalent of the manubrium, and as it is implanted on the blind end of the pneumatophore, such a view leads necessarily to the air-sack and gland being a development on the ex-umbral surface of the medusa-person. Chun and Woltereck, on the other hand, regard the stem as a stolo prolifer arising from the aboral pole, that is to say, from the ex-umbrella, similar to that which grows out from the ex-umbral surface of the embryo of the Narcomedusae and produces buds, a view which is certainly supported by the embryological evidence to be adduced shortly. In the siphosome the following types of appendages occur: i. Siphons or nutritive appendages, from which the order takes its name; never absent and usually present in great numbers (fig. 68, e). Each is a tube dilated at or towards the base and containing a mouth at its extremity, leading into a stomach placed in the dilatation already mentioned. The siphons have been compared to the manubrium of a medusa-individual, or to polyps, and hence are sometimes termed gastrozoids. 2. Palpons (fig. 68, g), present in some genera, especially in Physonectae; similar to the siphons but without a mouth, and purely tactile in function, hence sometimes termed dactylozoids. If a distal pore or aperture is present, it is excretory in function; such varieties have been termed " cystons " by Haeckel. n, Pneumatocyst. k, Nectocalyces (swimming bells). 1, Hydrophyllium(covering-piece). i, Generative medusiform person. g, Palpon with attached palpacle, h. e, Siphon with branched grappling tentacle, f. the siphosome, bearing apt, Stem. pendages which are nutritive, reproductive or simply protective in function. Divergent views have been held by different authors both as regards the nature of the cormus as a whole, and as regards the homologies of the different types of appendages borne by it. The general theories of Siphonophoran morphology are discussed 3. Tentacles (" Fangfaden "), always present, and implanted one the appendages are arranged as regularly recurrent cornidia along it, at the base of each siphon (fig. 68, f). The tentacles of siphonophores and the cormidia are then said to be "ordinate." In such cases the may reach a great length and have a complex structure. They may oldest cormidia, that is to say, those furthest from the nectosome, hear accessory filaments or tentilla (f'), covered thickly with batteties may become detached (like the segments of nematocysts, to which these organisms owe their great powers of or proglottides of a tape-worm) and offence and defence. swim off, each such detached cormidium 4. Palisades (" Tastfaden "), occurring together with palpons, one then becoming a small free cormus implanted at the base of each palpon (fig. 68, h). Each palpacle is which, in many cases, has been given an a tactile filament, very extensile, without accessory filaments or independent generic name. A cormidium nematocysts. may contain a single nutritive siphon 5. Bracts (" hydrophyllia "), occur in Calycophorida and some (" monogastric ") or several siphons Physophorida as scale-like appendages protecting other parts (fig. (" polygastric "): From G. H. Fowler, after G. 68, 1). The mesogloea is greatly developed in them and they are ' The following are some of the forms Cuvier often of very tough consistency. By Haeckel they of cormidia that occur: S S are considered homologous with the umbrella of a I. The eudoxome (Calycophorida), con- FIG. 71.-Upper sur- medusa. sisting of a bract, siphon, tentacle and face of Velella, showing 6. Gonostyles, appendages which produce by budding •gonophore; when free it is known as pneumatophore and sail. medusae or gonophores; like the blastostyles of a hydroid ! Eudoxiu. colony. In their most primitive form they are seen in 2. The ersaeome (Calycophorida), made up of the same appendages Velella as " gonosiphons,'" which possess mouths like as the preceding type but with the addition of a nectocalyx; when the ordinary sterile siphons and bud free medusae. In free termed Ersaea. other forms they have no mouths. They may he 3. The rhodalome of some Rhodalidae, consisting of siphon, tentacle branched, so-called " gonodendra," and amongst them and one or more gonophores. may occur special forms of palpons, " gonopalpons." 4. The athorome of Physophora, &c., consisting of siphon, tentacle, The gonostyles have been compared to the blastostyles one or more palpons with, palpacles, and one or more gonophores. of a hydroid colony, or to the manubrium of a 5. The crystallome of Anthemodes, &c., similar to the athorome but medusa which produces free or sessile medusa-buds. with the addition of a 7. Gonophores, produced either on the gonostyles group of bracts. already mentioned or budded, as in hydrocorallines, Embryology of the from the coenosarc, i.e. the stem (fig. 68, i.). They Siphonophora.—The fershow every transition between free medusae and tilized ovum gives rise, sporosacs, as already described for hydroid colonies. to a parenchymula, with Thus in Velella free medusae are produced, solid endoderm, which is which have been described as an inde- set free as a free-swimpendent genus of medusae, Chrysomltra. ming planula larva, in In other types the medusae may be set the manner already de- scribed (see HYDROZOA). The planula has its two extremities dissimilar (Bipolaria-larva). The subsequent development is slightly different ac-cording as the future cormus is headed by a pneumatophore (Physophorida, Cystophorida) or by a nectocalyx (Calycophorida). (i.) Physophorida, for example Halistemma (C. Chun, HYDROZOA {I]). The planula becomes elongated and broader towards one pole, at which a pit or invagination of the ectoderm arises. Next the pit closes up to form a vesicle with a pore, and. so gives rise to the pneumatophore. From the broader portion of From G. H. Fowler, after A. Agassiz, Lankester's Treatise on Zoology. the p 1 a n u 1 a a ri peripheral thirds of a half-section of the colony, the middle third being which becomes the omitted. The ectoderm is indicated by close hatching, the endoderm by first tentacle of FIG. 72.- A, Diphyes c¢mp¢nul¢t¢ light hatching, the mesogloea by thick black lines, the horny skeleton of the cormus. The B, a group of appendages (cormidium) of the pneumatophore and sail by dotting. endoderm of the the same Diphyes. (After C. Gegenbaur.) now BL, Blastostyle. M, Vedusoid gonophores. planula now a caviacty, a, Axis of the colony. o, Orifice o f C, Centradenia. P N, Primary central chamber, and m, Nectocalyx. nectocalyx. U, Palpon. PN', concentric chamber of and a t t h e c Sub-umbral cavity t, Bract. EC, Edge of colony prolonged he- the pneumatophore, showing m uth rower. s pole of nectocalyx. n, Siphon. yond the pneumatophore. an opening to the exterior, v, Radial canals of g, Gonophore. G, Cavity of the large central and a "trachea." giving rise to the primary siphon. nectocalyx. i, Tentacle. Thus from the original planula three appendages are, as it were, budded off, while the planula itself mostly gives rise to coenosarc, just as in some hydroids the planula is converted chiefly into hydrorhiza. (ii.) Calycophorida, for example, Muggiaea. The planula develops, on the whole, in a similar manner, but the ectodermal invagination arises, not at the pole of the planula, but on the side of its broader portion, and gives rise, not to a pneumatophore, but to a nectocalyx, the primary swimming bell or protocodon (" Fallschirm ") which is later thrown off and replaced by secondary swimming bells, metacodons, budded from the coenosarc. Lankester's Treatise on siphon. S, Sail. free in a mature condition as the so-called " genital swimmingg bells," comparable to the Globiceps of Penn¢ria. The most usual condition, however, is that in which sessile medusoid gonophores or sporosacs are produced. The various types of appendages described in the foregoing may be arranged in groups termed cormidia. In forms with a compact coenosarc such as Velella, Physalia, &c., the separate cormidia cannot be sharply distinguished, and such a condition is described technic-ally as one with ' scattered " cormidia. In forms in which, on the other hand, the coenosarc forms an elongated, tubular axis or stem, From a comparison of the two embryological types there can be no doubt on two points; first, that the pneumatophore and the protocodon are strictly homologous, and, therefore if the nectocalyx is comparable to the umbrella of a medusa, as seems obvious, the pneumatophore must be so too; secondly, that the coenosarcal axis arises from the ex-umbrella of the medusa and cannot he compared to a manubrium, but is strictly comparable to the " bud-spike of a Narcomedusan. Theories of Siphonophore Morphology.--The many theories that have been put forward as to the interpretation of the cormus and the various parts are set forth and discussed in the treatise of Y. Delage and E. Herouard (HYDROZOA [4]) and' more recently by R. Woltereck [59], and only a brief analysis can be given here. In the first place the cormus has been regarded as a single individual and its appendages as organs. This is the so-called " poly-organ " theory, especially connected with the name of Huxley; but it must be borne in mind that Huxley regarded all the forms produced, in any animal, between one egg-generation and the next, as constituting in the lump one single individual. Huxley, there-fore, considered a hydroid colony, for example, as a single individual, and each separate polyp or medusa budded from it as having the value of an organ and not of an individual. Hence Huxley's view is not so different from those held by other authors as it seems to be at first sight. In more recent years Woltereck [59] has supported Huxley's view of individuality, at the same time drawing a fine distinction between " individual " and " person." The individual is the product of sexual reproduction ; a person is an individual of lower rank, which may be produced asexually. A Siphonophore is regarded as a single individual composed of numerous zoids, budded from the primary zoid (siphon) produced from the planula. Any given zoid is a person-zoid if equivalent to the primary zoid, an organ-zoid if equivalent only to a part of it. Woltereck considers the siphonophores most nearly allied to the Narcomedusae, producing like the buds from an aboral stolon, the first bud being represented by the pneumatophore or protocodon, in different cases. Contrasting, in the second place, with the polyorgan theory are the various " polyperson " theories After C. Gegenbaur. which interpret the Siphonophore cormus as a colony composed of FIG.73.—Physophora hydrostatica. more or fewer individuals in or- a', Pneumatocyst. ganic union with one another. On t, Palpons. this interpretation there is still a, Axis of the colony. room for considerable divergence m, Nectocalyx. of opinion as regards detail. To o, Orifice of nectocalyx. begin with, it is not necessary on n, Siphon. the polyperson theory to regard g, Gonophore. each appendage as a distinct in- Tentacle. dividual; it is still possible to compare appendages with parts of an individual which have become separated from one another by a process of " dislocation of organs." Thus a bract may be regarded, with Haeckel, as a modified umbrella of a medusa, a siphon as its manubrium, and a tentacle as representing a medusan tentacle shifted in attachment from the margin to the sub-umbrella; or a siphon may be compared with a polyp, of which the single tentacle has become shifted so as to be attached to the coenosarc and so on. Some authors prefer, on the other hand, to regard every appendage as a separate individual, or at least as a portion of an individual, of which other portions have been lost or obliterated. A further divergence of opinion arises_ from differences in the interpretation of the persons composing the colony. It is possible to regard the cormus (i) as a colony of medusa-persons, (2) as a colony of polyp-persons, (3) as composed partly of one, partly of the other. It is sufficient here to mention briefly the views put forward on this point by C. Chun and E. Haeckel. Chian (HYDROZOA 11]) maintains the older views of Leuckart andClaus, according to which the cormus is to be compared to a floating hydroid colony. It may be regarded as derived from floating polyps similar to Nernopsis or Pelagohydra, which by budding produce a colony of polyps and also form medusa-buds. The polyp-individuals form the nutritive siphosome or trophosome. The medusa-buds are either fertile or sterile. If fertile they become free medusae or sessile gonophores. If sterile they remain attached and loco-motor in function, forming the nectosome, the pneumatophore and swimming-bells. Haeckel, on the other hand, is in accordance with Balfour in regarding a Siphonophore as a medusome, that is to say, as a colony composed of medusoid persons or organs entirely. Haeckel considers that the Siphonophores have two distinct ancestral lines of evolution : t. In the Disconanthae, i.e. in such forms as Velella, Porpita, &c., the ancestor was an eight-rayed medusa (Disconula) which acquired a pneumatophore as an ectodermal pit on the ex-umbrella, and in which the organs (manubrium, tentacles, &c.) became secondarily multiplied, just as they do in Gastroblasta as the result of incomplete fission. The nearest living allies of the ancestral Disconula are to be sought in the Pectyllidae. 2. In the Siphonanthae, i.e. in all other Siphonophores, the ancestral form was a Siphonula, a bilaterally symmetrical Anthomedusa After Haeckel, from Lankester's Treatise on Zoology. p, Pneumatophore. 1, Aurophore. s, Siphon. n, Nectocalyx. lo, Orifice of the aurophore. t, Tentacle. with a single long tentacle (cf. Corymorpha), which became displaced from the margin to the sub-umbrella. The Siphonula produced buds on the manubrium, as many Anthomedusae are known to do, and these by reduction or dislocation of parts gave rise to the various appendages of the colony. Thus the umbrella of the Siphonula became the protocodon, and its manubrium, the axis or stolon, which, by a process of dislocation of organs, escaped, as it were, from the sub-umbrella through a cleft and became secondarily attached to the ex-umbrella. It must be pointed out that, however probable Haeckel's theory may be in other respects, there is not the slightest evidence for any such cleft in the umbrella having been present at any time, and that the embryological evidence, as already pointed out, is all against any homology between the stem anti a manubrium, since the primary siphon does not become the stem, which arises from the ex-umbral side of the protocodon and is strictly comparable to a stolon. Classification.—The Siphonophora may be divided, following Delage and Herouard, into four sub-orders: I. CHONDROPHORIDA (Disconectae Haeckel, Tracheophysae Chun). With an apical chambered pneumatophore, from which tracheal tubes may take origin (fig. 7o) ; no nectocalyces or bracts; appendages all on the lower side of the pneumatophore arising from a compact coenosarc, and consisting of a central principal siphon, surrounded by gonosiphons, and these again by tentacles. Three families: (1) Discalidae, for Discalia and allied genera, deep-sea forms not well known; (2) Porpitidae for the familiar genus Porpita (fig. 69) and its allies; and (3) Velellidae, represented by the well-known genus Velella (figs. 70, 71), common in the Mediterranean and other seas. II. CALYCOPHORIDA (Calyconectae, Haeckel). Without pneumatophore, with one, two, rarely more nectocalyces. Three families: (I) Monophyidae, with a single nectocalyx; examples Muggiaea, sometimes found in British seas, Sphaeronectes, &c.; (2) Diphyidae, with two nectocalyces; examples Diphyes (fig. 72), Praya, Abyla, &c.; and 3) Polyphyidae, with numerous nectocalyces; example Hippopodius, Stephanophyes and other genera. d' MY From G H. Fowler, modified after C. Cuvier and E. Haeckel, Lankester's Treatise on Zoology. A great number' of families and genera are referred to this group, amongst which may be mentioned specially—(i) Agalmidae, containing the genera Stephanomia, Agalma, Antheniodes, Halistemma, &c.; (2) Apolemidae, with the genus Apolemia and its allies; (3) Forskalisdae, with Forskalia and allied forms; (4) Physophoridae, for Physophora (fig. 73) and other genera, (5) Anthophysidae, for Anthophysa, Athorybia, &c.; and lastly the two families (6) Rhodalidae and (7) Stephalidae (fig. 74), constituting the group Auronectae of Haeckel. The Auronectae are peculiar deep-sea forms, little known except from Haeckel's descriptions, in which the large pneumatophore has a peculiar duct, termed the aurophore, placed on its lower side in the midst of a circle of swimming-bells. IV. CYSTOPHORIDA (Cystonectae, Haeckel). With a very large pneumatophore not divided into chambers, but without nectocalyces or bracts. 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