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Originally appearing in Volume V27, Page 392 of the 1911 Encyclopedia Britannica.
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ORDER III.—ASCIDIACEA Fixed or free-swimming simple or compound Ascidians which in the adult are never provided with a tail and have no trace of a notochord. The free-swimming forms are colonies, the Ascidiacea. simple Ascidians being always fixed. The test is perma- nent and well developed; as a rule it increases with the age of the individual. The branchial sac is large and well developed. Its walls are perforated by numerous slits (stigmata) opening into the peribranchial cavity, which communicates with the exterior by the atria'_ aperture. Many of the forms reproduce by gemination, and in most of them the sexually-produced embryo develops into a tailed larva. The Ascidiacea includes three groups—the simple Ascidians, the compound Ascidians and the free-swimming colonial Pyrosoma. Sub-Order r.—Ascidiae simplices. Fixed Ascidians which are solitary and very rarely reproduce by gemmation; if colonies are formed, the members are not buried in a common investing mass, but each has a distinct test Simple of its own. No strict line of demarcation can be drawn Ai glens. between the simple and the compound Ascidians, and one of the families of the former group, the Clavelinidae (the social Ascidians), forms a transition from the typical simple forms, which never reproduce by gemmation, to the compound forms, which always do. The Ascidiae Simplices may be divided into the following families: Family I., Clavelinidae.—Simple Ascidians which reproduce by gemmation to form small colonies in which each ascidiozooid has a distinct test, but all are connected by a common blood system, and by prolongations of " epicardiac tubes " from the branchial sacs. Buds formed on stolons which are vascular outgrowths from the posterior end of the body, containing prolongations from the ectoderm, mesoderm and endoderm of the ascidiozooid. Branchial sac not folded; internal longitudinal bars usually absent; stigmata straight; tentacles simple. This family contains, amongst others, the following three genera: Ecteinascidia (Herdman), with internal longitudinal bars in branchial sac; Clavelina (Savigny), with intestine extending behind branchial sac; and Perophora (Wiegmann), with intestine alongside branchial sac. Family II., Ascidiidae.—Solitary fixed Ascidians with gelatinous test; branchial aperture usually eight-lobed, atrial aperture usually six-lobed. Branchial sac not folded; internal longitudinal bars usually present ; stigmata straight or curved ; tentacles simple. This family is divided into three sections: Sub-family r, Hypobythinae.—Branchial sac with no internal longitudinal bars. One genus, Hypobythius (Moseley). Sub family 2, Ascidinae.—Stigmata straight. Many genera, of which the following are the more important: Ciona (Fleming), dorsal languets present; Ascidia (Linnaeus, = Phallusia, Savigny), dorsal lamina present (see figs. 1 to To); Rhodosoma (Ehrenberg), anterior part of test modified to form operculum; Abyssascidia (Herdman), intestine on right side of branchial sac. Sub family 3, Corellinae.—Stigmata curved. Three chief genera: Corella (Alder and Hancock), test gelatinous, body sessile; Corynascidia (Herdman), test gelatinous, body pedunculated; Chelyosoma (Brod. and Sow.), test modified into horny plates. Family III., Cynthiidae.—Solitary fixed Ascidians, usually with leathery test; branchial and atrial apertures both four-lobed. Branchial sac longitudinally folded (fig. 26); stigmata straight; tentacles simple or compound. This family is divided into three sections: Sub-family r, Styelinae.—Not more than four folds on each side of branchial sac (fig. 26, S) tentacles simple. The more important genera are: Styela (Macleay), stigmata normal, and Bathyoncus (Herdman), stigmata absent or modified. br/ i 1 (After Hardman, " Challenger " Report.) A, Entire body, natural size. B, Part of branchial sac magnified. br f, Slight fold of branchial sac. at, Atrial aperture. i Internal longitudinal bar. br, Branchial aperture. mh, Mesh. ped, Peduncle. sp, Calcareousspiculesinvessels. tr, Transverse vessels. Sub family 2, Cynthinae.--More than eight folds in branchial sac (fig. 26, C) ; tentacles compound; body sessile. The chief genus is Cynthia (Savigny), with a large number of species. Sub-family 3, Bolteninae.—More than eight folds in branchial sac; tentacles compound; body pedunculated (fig. 25, A). The chief genera are: Boltenia (Savigny), branchial aperture four-lobed, stigmata normal; and Culeolus (Herdman), branchial aperture with less than four lobes, stigmata absent or modified (fig. 25, B). This last is a deep-sea genus discovered by the " Challenger " expedition (see 17). Family I V., Molgulidae.—Solitary Ascidians, sometimes not fixed; branchial aperture six-lobed, atrial four-lobed. Test usually incrusted with sand. Branchial sac longitudinally folded; stigmata more or less curved, usually arranged in spirals; tentacles compound. The chief genera are: Molgula (Forbes), with distinct folds in the branchial sac, and Eugyra (Ald. and Hanc.), with no distinct folds, but merely broad internal longitudinal bars in the branchial sac. In some of the Molgulidae (genus Anurella, Lacaze-Duthiers, 2o) the embryo (fig. 14, M) does not become converted into a tailed larva, the development being direct, without metamorphosis. The embryo when hatched assumes gradually the adult structure, and never shows the features characteristic of larval Ascidians, such as the urochord and the median sense-organs. Bourne has described an aberrant Molgulid, Oligotrema, from the Loyalty Islands, with a reduced branchial sac and enlarged pinnate muscular branchial lobes, apparently used for catching food! A. S. A, Unfolded type. S, Slyela, with four folds on each side. C, Cynthia, with eight folds on one side and seven on the other. D.L., Dorsal lamina; End, endostyle; I, II, &c., folds. C CI cr.) P, Plain. F, Folded. A, Areolated. i, Intestine; cc, oesophagus; st, stomach. Figs. 26 and 27 illustrate some details of structure of branchial sac and of stomach in various simple and compound Ascidians, which are made use of in classification, and in the definitions of genera and larger groups. Sub-Order 2.-Ascidiae Compositae. Fixed Ascidians which reproduce by gemmation, so as to form colonies in which the ascidiozooids are buried in a common invest-Compound ing mass and have no separate tests. This is probably Ascidians. a somewhat artificial assemblage formed of two or three groups of Ascidians which produce colonies in which the ascidiozooids are so intimately united that they possess a common test or investing mass. This is the only character which distinguishes them from the Clavelinidae, but the property of reproducing by gemmation separates them from the rest of the Ascidiae Simplices. The Ascidiae Compositae may be divided into seven families, which fall into two well-marked groups: (I) the Chalarosomata, including the first five families, with extended body, divided into two or three regions, and more nearly related to the Clavelinidae; and (2) the Pectosomata, including the Botryllidae and Polystyelidae, with a compact body, not divided into regions, and evidently related to the Cynthiidae amongst simple Ascidians. Family I., Distomidae.—Ascidiozooids divided into two regions,thorax and abdomen; testes numerous; vas deferens not spirally coiled. The chief genera are : Distoma (Gaertner) ; Distaplia (Della Valle) ; Colella (Herdman), forming a pedunculated colony (see fig. 28, A) in which the ascidiozooids develop incubatory pouches, connected with the peribranchial cavity, in which the embryos undergo their development (17) ; and Chondrostachys (Macdonald). Family II., Coelocormidae.—Colony not fixed, having a large axial cavity with a terminal aperture. Branchial apertures five-lobed. This includes one species, Coelocormus huxleyi (Herdman), which is, in some respects, a transition form between the ordinary compound Ascidians (e.g. Distomidae) and the Ascidiae Luciae (Pyrosoma). Family III., Didemnidae.—Colony usually thin and incrusting test containing stellate calcareous spicules. Testis single, large; c (After Herdman, "Challenger" Report.) A, Colella quoyi. D, Botryllus, showing arrangement B, Leptoclinum neglectum. of ascidiozooids in circular C, Pharyngodictyon mirabile. systems, each of which has a central common cloaca. vas deferens spirally coiled. The chief genera are—Didemnum (Savigny), in which the colony is thick and fleshy and there are only three rows of stigmata on each side of the branchial sac; and Leptoclinum (Milne-Edwards), in which the colony is thin and incrusting (fig. 28, B) and there are four rows of stigmata on each side of the branchial sac. Family IV., Diplosomidae.—Test reduced in amount, rarely containing spicules. Vas deferens not spirally coiled. In Diplosoma (Macdonald), the most important genus, the larva is gemmiparous. Family V., Polyclinidae.-Ascidiozooids divided into three regions—thorax, abdomen and post-abdomen. Testes numerous; vas deferens not spirally coiled. The chief genera are: Pharyngodictyon (Herdman), with stigmata absent or modified, containing one species, Ph. mirabile (fig. 28, C), the only compound Ascidian known from a depth of moo fathoms; Polyclinum (Savigny), with a smooth-walled stomach; Aplidium (Savigny), with the stomach wall longitudinally folded (fig. 27); and Amaroucium (Milne-Edwards), in which the ascidiozooid has a long post-abdomen and a large atrial languet, Family VI., Botryllidae.—Ascidiozooids having the intestine and reproductive organs alongside the branchial sac. Dorsal lamina present; internal longitudinal bars present in branchial sac. The chief genera are: Botryllus (Gaertn. and Pall.), with simple stellate systems (fig. 28, D), and Botrylloides(Milne-Edwards), with elongated or ramified systems. It is well known that in the family Botryllidae, amongst compound Ascidians, the ectodermal vessels containing (After Pizon.) B4 blood, which ramify through the common test and serve to connect the vascular systems of the various members of the colony, have numerous large ovate dilatations, the ampullae, upon their terminal twigs (fig. 29). Various functions have been assigned to these ampullae in the past, and Bancroft has shown that in addition to acting as storage reservoirs for blood, organs for the secretion of test matrix, and accessory organs of respiration, they are also organs for blood propulsion. The ampullae execute co-ordinated pulsations, the co-ordination being due to variations in the blood-pressure. It was actually found that the ampullae could keep up the circulation for some time in a portion of a colony independently of the hearts of the ascidiozooids. All the hearts in a colony of Botryllus contract simultaneously and in the same direction. The reversal of the circulation may be regarded as due to the engorgement of the ampullae in the superficial parts of the colony. These when distended overcome the resistance of the heart's action, and cause it to stop and then reverse. Family VII., Polystyelidae.—Ascidiozooids not. grouped in systems. Branchial and atrial apertures four-lobed. Branchial sac may be folded; internal longitudinal bars present. The chief genera are: Thylacium (Carus), with ascidiozooids projecting above general surface of Colony ;Goodsiria (Cunningham), with ascidiozooids completely imbedded in investing mass; and Chorizocormus (Herd-man), with ascidiozooids united in little groups which are connected by stolons. Several of the species show transitions between the other Polystyelidae and the Styelinae amongst simple Ascidians. Gemmation and Growth of Colonies.—A number of new observations have been made in recent years upon the budding of compound Ascidians, some of which are very puzzling and contradictory in their results. Metschnikoff, Kowalevsky, Giard, Hjort, Pizon, Seeliger, Ritter, van Beneden and Julin have all in turn added to our knowledge of the details of development and life-history, of the various processes of gemmation and of the formation of colonies. It is impossible as yet to reconcile all the conflicting accounts, but the following points at least seem pretty clear. Gemmation may be very different in its details in closely related compound Ascidians. There are, however, two main types of budding, to one or other of which most of the described methods may be referred. There is first the "stolonial or " epicardiac " type, seen in the Chalarosomata, typically in Distomidae and Polyclinidae, and comparable with the gemmation in Clavelinidae, Pyrosomidae and Thaliacea outside this group. Secondly, there is the " parietal " or "peribranchial " type, seen in the Pectosomata. typically in the Botryllidae. The remarkable process of gemination. seen in the families Didemnidae and Diplosomidae may probably be regarded as a modification of the stolonial type. The double embryo in the Diplosomidae is probably to be interpreted as precocious budding (rather than as embryonic fission), due to acceleration in development (tachygenesis). The type of budding, and even details such as the length of the stolon, have much to do with differences in the nature and appearance of the colonies produced. The stolon, which has a wall continuous with the body-wall of the parent, contains an endodermal element in the form of the so-called " epicardium," and also a prolongation of the ovary,,or at least a string of migrating germ-cells, so that the reproductive elements are also handed on. Still, it is clear from recent researchesand attempts to explain budding in Ascidians as a process of re-generation, by which the organs of the parent or their germ-layers give rise to the corresponding organs in the bud, have signally failed. Figs. 29 and 30 show the buds in the Botryllidae, after Pizon, who has followed day by day the changes of growth in young colonies of Botryllidae, tracing the rise of successive generations of buds and the degeneration of their parents. The buds are parietal, arising from the walls of the peribranchial cavities (fig. 29), and at an early period they acquire the structure shown in fig. 30, where there are two vesicles undergoing further subdivision and differentiation, but investigators still differ as to whether the inner, which gives rise to the branchial sac and alimentary canal, is not produced along with the outer from the ectoderm of the parent. A remarkable case of polymorphism has been found by M. Caullery in the buds of the compound Ascidian Colella. Some of the buds have an abundant store of reserve materials in their Reducouter layer of cells, while others are without this supply. tionprboy The former are placed deeply in the stalk, develop slowly, a' and probably serve to regenerate the colony when the and the head portion has been removed or has died down. In Formation these cases where the ectoderm has taken on the function ofColontes. of storing the reserve material, it is found that all the organs of the bud are formed from the cells of the endodermic vesicle. The first ascidiozooid of the colony produced by the tailed larva does not form sexual reproductive organs, but reproduces by gemmation so as to make a colony. Thus there is alternation of generations in the life-history. In the most completely formed. colonies (e.g. Botryllus) the ascidiozooids are arranged in groups' (systems or coenobii), and in each system are placed with their. atrial apertures towards one another, and all communicating with a common cloacalcavity which opens to the exterior in the centre of the system (fig. 28, D). Sub-Order 3.-Ascidiae Luciae. Free-swimming pelagic colonies having the form of a hollow cylinder closed at one end. The ascidiozooids forming the colony are em-bedded in the common Aacldlae test in such a manner Guc-ae. that the branchial apertures open on the outer surface and the atrial apertures on the inner surface next to the central cavity of the colony. The ascidiozooids are produced by gemmation from a rudimentary larva (the cyathozooid) developed sexually. This sub-order includes a single family, the Pyrosomidae, containing one well marked structure genus, Pyrosoma (Peron), with half a soma. dozen species. They are found swimming near the surface of the sea, chiefly in tropical latitudes, and are brilliantly phosphorescent. A fully developedPyrosoma colony may be from an inch or two to upwards of twelve feet in length. The shape of the colony is seen in fig. 31. Ii tapers slightly towards the closed end, which is rounded. The opening at the opposite end is reduced in size by the presence of a membranous prolongation of the common test (fig. 31, B). The branchial apertures of the ascidiozooids are placed upon short papillae projecting from the general surface, and most' of the ascidiozooids have long conical processes of the test projecting FIG. 31.—Pyrosoma elegans. outwards beyond their branchial (Natural size.) apertures (figs. 31, 32 and 33)• A, Side view of entire colony. There is only a single layer of B, End view of open extremity. ascidiozooids in the Pyrosoma colony, as all the fully developed ascidiozooids are placed with their antero-posterior axes at right angles to the surface and communicate by their atrial apertures with the central cavity of the colony (fig. 32). Their dorsal surfaces are turned towards the open end of the colony. The more important points in the structure of the ascidiozoeid of Pyrosoma are shown in fig. 33. A circle of tentacles, of which one, placed ventrally (fig. 33, tn), is larger than the rest, is found just Inside the branchial aperture. From this point a wide cavity, with a few circularly placed muscle bands running round its walls, leads back to the large branchial sac, which occupies the greater part of the body. The stigmata are elongated transversely and crossed by internal' longitudinal bars. The dorsal lamina is represented by a series of eight languets (l). The nerve ganglion (on which is placed a small pigmented sense organ), the subneural gland, the dorsal tubercle, the peripharyngeal (After Pizon.) ov, Dorsal tube. gh, Germ cells. m, Mesoderm cells. ect, Ectoderm. that the development of the bud (blastozooid) and that of the embryo (oozooid) do not proceed along parallel lines. It is impossible to harmonize the facts of gemmation with the germ-layer theory, bands, and the endostyle are placed in the usual positions. On each side of the anterior end of the branchial sac, close to the peripharyngeal bands, is a mass of rounded gland cells which are the source of the phosphorescence. The alimentary canal is placed at, Atrial apertures. em, Embryos in various stages. br, Branchial apertures. t, Test. asc, Young ascidiozooid of a future &p, Processes of test. colony produced by budding from cy, cyathozooid. br.s, Branchial sac.,Young ascidiozooid. posteriorly to the branchial sac, and the anus opens into a large peribranchial (or atrial) cavity, of which only the median posterior part is shown ( in fig. 33. The reproductive organs are developed in a diverticulum of the peribranchial cavity, and consist of a lobed testis and a single ovum at a time. The development takes place in a part of the peribranchial cavity (fig. 32, em). The segmentation Development is meroblastic, and an elongated embryo is formed on ofPyro- the surface of a mass of yolk. The embryo, after the soma. formation of an alimentary cavity, a tubular nervous system, and a pair of laterally placed atrial tubes, divides into an anterior and a posterior part. The anterior part then segments into four pieces, which afterwards develop into the first ascichozooids of the colony, while the posterior part remains in a rudimentary condition, as the " cyathozooid "; it eventually atrophies. As the four ascidiozooids increase in size, they grow round the cyathozooid and soon encircle it (fig. 32, asc and cy). The cyathozooid "bsorbs the nourishing yolk upon which it lies, and distributes it lei (Partly after Keferstein.) Lettering as before. c.m, Cellular mass, the seat of m.b, Muscle band. phosphorescence., Subneural gland. c.m', Posterior cellular mass. pig, Pigment spot on ganglion. g.s, Gemmiparous stolon. t.p, Process of test. to the ascidiozooids by means of a heart and system of vessels which have been meanwhile formed. When the cyathozooid atrophies and is absorbed, its original atrial aperture remains and deepens to become the central cavity of the young colony, which now consists of four ascidiozooids placed in a ring, around where the cyathozooid was, and enveloped in a common test. The colony gradually increases by the formation of buds from these four original ascidiozooids. PHYLOGENY The accompanying diagram (fig. 34) shows graphically the probable origin and course of evolution of the various groups of Tunicata, and therefore exhibits their relations to one another much more correctly than any system of linealogeny classification can do. The ancestral Proto-Tunicata are here regarded as an offshoot from the Proto-Chordata--the common astq ""sa,ios ma FIG. 34. ancestors of the Tunicata (Urochorda), Amphioxus (Cephalochorda) and the Vertebrata. The ancestral Tunicata were probably free-swimming forms, not very unlike the existing Appendiculariidae, and are represented in the life-history of nearly all sections of the Tunicata by the tailed larval stage. The Larvacea are the first offshoot from the ancestral forms which gave rise to the two lines of descendants, the Proto-Thaliacea and the Proto-Ascidiacea. The Proto-Thaliacea then split into the ancestors of the existing Cyclomyaria and Hemimyaria. The Proto-Ascidiacea gave up their pelagic mode of life and became fixed. This ancestral process is repeated at the present day when the free-swimming larva of the simple and compound Ascidians becomes attached. The Proto-Ascidiacea, after the change, are probably most nearly represented by the existing genus Clavelina. They have given rise directly or indirectly to the various groups of simple and compound Ascidians and the Pyrosomidae. These groups form two lines, which appear to have diverged close to the position of the family Clavelinidae. The one line leads to the more typical compound Ascidians, and includes the Polyclinidae, Distomidae, Didemnidae, Diplosomidae, Coelocormidae, and finally the Ascidiae Luciae or Salpiformes. The second line gave rise to the simple Ascidians, and to the Botryllidae and Polystyelidae, which are, therefore, not closely allied to the other compound Ascidians. The later Proto-Ascidiacea were probably colonial forms, and gemmation was retained by the Clavelinidae and by the typical compound Ascidians (Distomidae; &c.) derived from them. The power of forming colonies by budding was lost, however, by the primitive simple Ascidians, and must, therefore, have been regained independently by the ancestral forms of the Botryllidae and the Polystyelidae. If this is a correct inter-• pretation of the course of evolution of the Tunicata, we arrive at the following important conclusions. (I) The Tunicata, as a whole, form a degenerate branch of the Proto-Chordata; (2) the Ascidiae Luciae (Pyrosoma) are much more closely related to the typical compound Ascidians than to the other pelagic Tunicata, viz. the Larvacea and the Thaliacea; and (3) the Ascidiae Compositae,form a polyphyletic group, the sections of which have arisen at several distinct points from the ancestral simple Ascidians. ' By Dohrn and others their point of origin is placed considerably farther up on the stem of the Chordata, thus causing the Tunicata to be regarded as very degenerate Vertebrata (see 31). ems (Partly after Savigny.) showing arrangement of ascidiozooids, magnified. of Miiller's Archiv (1852); (12) Kupffer, Arch. f. mikr. Anat. (1869, 1872) ; (13) Giard, Etude d. tray. embryolog. d. Tun., &c.," in Arch. zool. exper. (1872), vol. i.; (14) Fol, Etudes sur les appendiculaires du detroit de Messine," in Mem. soc. phys. hist. nat. Geneve, vol. xxi.; (15) Giard, " Recherches s. 1. Asc. Comp.," in Arch. zool. exper. (1872), vol. i.; (16) Von Drasche, Die Synascidien der Bucht von Rovigno (Vienna, 1883); (17) Herdman, " Report upon the Tunicata of the ' Challenger ' Expedition," pt. i. in Zool. Chall." Exp. (1882), vol. vi.; pt. ii. in Zool. " Chall." Exp. (1886), vol. xiv.; pt. iii. in Zool. " Chall." Exp. (1889), vol. xxvii.; (18) Alder and Hancock, in Ann. Mug. Nat. Hist. (1863, 1870) ; (19) Heller, Untersuch. u. d. Tunic. d. Adriat. Meeres," in Denkschr. d. k. Akad. WisS. (1875—1877); (20) Lacaze-Duthiers, "Asc. simp. d. cotes d. 1. Manche," in Arch. zool. expel'. (18i4, 1877); (21) Traustedt, in Vidensk. medd. naturh. For. (Copenhagen, 1881—1884); (22) Herd-man, " Notes on British Tunicata, &c., in Journ. Linn. Soc. Zool. (188o), vol. xv. ; (23) Ussoff, in Proc. imp. soc. nat. hist. (Moscow, 1876), vol. xviii.; (24) Julin, " Rech. s. l'org. d. asc. simp.," in Arch. d. biol. (1881), vol. ii.; (25) Brooks, " Development of Sal pa," in Bull. Mus. Comp. Zool. iii. 291 (Harvard) ; (26) Salensky, Ztschr. f. wiss. Zool. (1877); (27) Barrois, Journ. d. l'anat. et phys. (1885), vol. xxi.; (28) Uljanin, Fauna, &c., d. Golfes von Neapel (1884), vol. x.; (29) Moseley, " On Deep-sea Ascid," in Trans. Linn. Soc. (1876), 2nd series, vol. i.; (30) E. van Beneden and Julin, " Morph. d. Tuniciers," in Arch. d. Biol. (1886), vol. vi.; (31) Dohrn, " Studien zur Urgesch. der Wirbelth." in Mitth. zool. Stat. Neapel; (32) Herdman, " Revised Classification," Journ. Linn. Soc. (1891), vol. xxiii.; (33) Herdman, Descriptive Catalogue of Australian Tunicata (1899); (34) Brooks, The Genus Sal pa (1893) ; (35) Seeliger, Bronn's Thier-Reich Tunicata. (W. A. HE.)
End of Article: ORDER III

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