ASCIDIAE COMPOSITAE BOTRYLLIDAE, ETC. 



3 9 



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.lf), which 

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



FIG. 53. Two "systems" from a FIG. 54. Goodsiria placenta, Herdman, 



colony of Botryllus violaceus, 

 M.-Edw. d, Common cloaca of a 

 system ; or, branchial apertures of 

 ascidiozooids, magnified. (After H. 

 Milne-Edwards.) 



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

 of colony showing ascidiozooids. (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. 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 



