GROUP B. _HOLOSOMATA_. Body short, compact, with viscera by the side of branchial sac; budding parietal FAM. 6. BOTRYLLIDAE.—Ascidiozooids grouped in systems round common cloacal apertures (Fig. 53). Ascidiozooids having the intestine and reproductive organs by the side of the branchial sac (Fig. 46, A, p. 82). Dorsal lamina and internal longitudinal bars present in the branchial sac. Neural gland, as in Cynthiidae, dorsal to the ganglion in place of ventral as in the majority of Tunicata. The chief genera are—_Botryllus_, Gaertn. and Pall, with simple stellate systems (Fig. 53), and _Botrylloides_, Milne-Edwards, with elongated or ramified systems. There are {89}many species of both these genera, which form brilliantly coloured fleshy crusts under stones and on sea-weeds at low tide. They are amongst the commonest and the most beautiful of British Ascidians. Both genera contain species remarkable for the rich profusion of ectodermal "vessels" which ramify and anastomose in the colonial test. On the margins of the colony these vessels end in knob-like dilatations, the ampullae (Fig. 46, A, _t.k_), which are said by Bancroft to pulsate rhythmically, and so aid in keeping up the colonial circulation. They are also storage reservoirs for the blood, doubtless help in respiration, and are organs for the secretion of the test-matrix. [Illustration: FIG. 53.—Two "systems" from a colony of _Botryllus violaceus_, M.-Edw. _cl_, Common cloaca of a system; _or_, branchial apertures of ascidiozooids, magnified. (After H. Milne-Edwards.) FIG. 54.—_Goodsiria placenta_, Herdman. A, Colony (half nat. size); B, section of colony showing ascidiozooids. (After Herdman, from _Challenger Reports_.)] FAM. 7. 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 the ascidiozooids projecting above the general surface of the colony; _Goodsiria_, Cunningham, with the ascidiozooids completely imbedded in the investing mass (Fig. 54); and _Chorizocormus_, Herdman, with the ascidiozooids {90}united in little groups which are connected by stolons. The last genus contains one species, _Ch. reticulatus_, in some respects a transition-form between the other Polystyelidae and the Styelinae amongst Simple Ascidians. BUDDING IN HOLOSOMATA—In the Polystyelidae, according to Ritter,[102] the budding is of the same type as in Botryllidae, the bud arising in each case from the lateral body-wall of the parent.