TUNICATA. 



97 



The Compound Ascidians include seven families which are 

 characterised chiefly by the method of bud formation, and by the 

 arrangement of the organs into one, two, or three regions. 



It is only possible, from limits of space, to refer to a few interest- 

 ing forms. 



The species of Botryllus are those most commonly met with. 

 They form richly coloured gelatinous incrustations on rocks and sea- 

 weeds. B. violaceus (Figs. 16, 17, and 18 D) is blue with white lines ; 

 B. umaragdiis, green ; B. marionis, brown with white and carmine ; B. 



Fig. 16. 



.ill "I -1 



■r^ r I IP I. / . 



I '■If "I y ( 



Botryllus violaceus on seaweed. (After H. Milne-Edwards.) 



castaneus, purple, and so on. The individuals are arranged in circular 

 systems with the branchial orifices round the circumference and the 

 atrial orifices opening into a common central cavity (Fig. 17), the 

 whole colony being composed of groups of systems. 



The exhibited specimen of B. violaceus was grown in the tanks 

 of the Biological Station at Plymouth. The red specimen of B. 

 aurolineatus, from Naples, shows well the branchial and cloacal 

 orifices. In Botrylloides, the individuals form elliptical or elongated 

 systems. 



GoUlla thomsoni was obtained near the Philippines at a depth 

 of 10 fathoms. The specimen, which is about 7 inches in length, 



H 



