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 IV., 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, 20) 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!
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Fig. 26.—Diagrams showing Transverse Sections of Typical Branchial Sacs. |
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A, Unfolded type. S, Styela, with four folds on each side. |
| D.L., Dorsal lamina; End, endostyle; I, II, &c., folds. |
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Fig. 27.—Types of Stomach amongst Compound Ascidians. P, Plain.F, Folded.A, Areolated. i, intestine;œ, 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 investing Compound Ascidians. mass and have no separate tests. This is probably 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: (1) 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 incubator 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, Coelocormas huxleyi (Herdman), which is, in some respects, a transition form between the ordinary compound Ascidians (e.g. Distomidae) and the Ascidiae Luciae (Pyrosoma).
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(After Herdman, “Challenger” Report.) | |||||||||
| Fig. 28.—Colonies of Ascidiae Compositae. (Natural size.) | |||||||||
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Family III., Didemnidae.—Colony usually thin and in crusting test containing stellate calcareous spicules. Testis single, large; 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 1000 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.
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(After Pizon.) |
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Fig. 29.—Young Colony of Botryllus, showing Buds and Ampullae. |
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
