ASCIDIAE COMPOSITAE — BOTRYLLIDAE, ETC. 



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, tl^, which 

 are said by Bancroft to pulsate rhythmically, and so aid in keep- 



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Fig. 53. — Two "systems" from a 

 colony of Botryllus violaceus, 

 M.-Edw. cZ, Common cloaca of a 

 system ; or, branchial apertures of 

 ascldiozooids, magnified. (After H. 

 Milne-Edwards.) 



Fig. 64. — Goodsiria placenta, Herdman. 

 A, Colony (half nat. size) ; B, section 

 of colony showing ascldiozooids. (After 

 Herdman, from Challenger Reports.) 



ing 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. 



Fam. 7. Polystyelidae. — Ascldiozooids not grouped in 

 systems ; brandiial and atrial apertures four-lobed ; branchial sac 

 may be folded; internal longitudinal bars present. The chief 

 genera are — Thylacium, Carus, with the ascldiozooids projecting 

 above the general surface of the colony ; Goodsiria,, Cunningham, 

 with the ascldiozooids completely imbedded in the investing mass 

 (Fig. 54) ; and Chorizocormus, Herdman, with the ascldiozooids 



