[Illustration: FIG. 47.—A, Colony of _Colella pedunculata_, Q. and G., nat. size: _a_, zone of buds; _b_, zone of young ascidiozooids; _c_, zone of reproducing adults; _d_, old decaying adults and incubatory pouches with larvae. B, Ascidiozooid, with incubatory pouch enlarged: _At_, atrial aperture; _Br_, branchial aperture; _emb_, embryos; _end_, endostyle; _ep.c_, epicardium; _inc.p_, incubatory pouch; _od_, oviduct; _od′_, its prolongation into _inc.p_; _od″_ its termination at tip of _inc.p_; _ov_, ovary; _p.br_, peribranchial opening of _inc.p_; _st_, stomach.] FAM. 1. DISTOMATIDAE.—Ascidiozooids divided into two regions, a thorax, containing the branchial sac, and an abdomen, with the remaining viscera (Fig. 47, B); testes numerous; vas deferens not spirally coiled. The chief genera are—_Distoma_, Gaertner, with some British species; _Chondrostachys_, Macdonald, _Cystodytes_, v. Drasche, with calcareous plate-like spicules in the test (Fig. 50, A); _Distaplia_, Della Valle, and _Colella_, Herdman, forming a pedunculated colony (Fig. 47, A), in which the ascidiozooids (Fig. 47, B) are provided with large incubatory pouches, opening from the peribranchial cavity, but also connected, as Bancroft[101] has recently shown, with the end of the oviduct (see Fig. 47, B). In these pouches the embryos undergo their development, and are set free by the decay of the top of the colony. The stolons pass from the ascidiozooids in the upper part of the colony down into the stalk, and there produce buds which gradually work up to the top of the stalk, where they take their places as young ascidiozooids. At the top of the colony the old ascidiozooids die and are removed (see Fig. 47, A). Caullery has shown that in {86}this genus there may be dimorphism in the buds, some of them placed deeply in the stalk having a large amount of reserve food-matter in their ectoderm, and remaining dormant until required to regenerate the "head" or upper part of the colony when it is lost. This genus was made known by the "Challenger" expedition. The species are mostly tropical, or from southern seas. FAM. 2. 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._ Distomatidae) and the Ascidiae Luciae (_Pyrosoma_, see p. 90). [Illustration: FIG. 48.—Transverse section of the abdomen of a Distomid. _bl.s_, Blood-sinus; _ec_, ectoderm; _ep.c_, epicardium; _gl_, intestinal glands; _h_, heart; _i_, intestine; _l.m_, longitudinal muscles; _mes_, mesoderm; _o.d_, oviduct; _p.c_, pericardium; _st_, stomach; _v.d_, vas deferens. (After Seeliger.)]