III 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, 4./:), which 
are said by Bancroft to pulsate rhythmically, and so aid in keep- 
Fig. 53.—Two “systems” from a Fic. 54.—Goodsiria placenta, Herdman. 
colony of  Botryllus violaceus, A, Colony (half nat. size); B, section 
M.-Edw. c/, Common cloaca of a of colony showing ascidiozooids. - (After 
system ; or, branchial apertures of Herdman, from Challenger Reports.) 
ascidiozooids, magnified. (After H. 
Milne-Edwards.) 
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—TZhylacium, 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 
