TUNICATA. 93 



observe the tadpoles become fixed and develop into fixed Ascidians. 

 He calls the active little swimming larvge " Spinulas," from their 

 resemblance to small pins. 



The family Ascidiidce includes forms with a gelatinous or 

 cartilaginous test ; the branchial and atrial orifices usually have 8 

 and 6 lobes respectively ; the branchial sac is without folds. 



Ascidia menhda, described above, belongs to this family. 



Ghelyosoma is characterised by the test forming tortoise-like 

 horny plates on the upper surface. The exhibited specimen of 



~f- 



Ghelyosoma macleayanum, slightly enlarged. 

 a, branchial ; b, atrial orifice. 



G. macleayanum (Fig. 12) comes from Greenland ; the upper hemi- 

 spherical part of the test is divided into 8 plates ; the branchial and 

 atrial orifices are situated in the joints between the plates. 



The fine specimen of Phallusia mammillata from Naples consists 

 of several individuals partly fused together ; the branchial and 

 atrial orifices are wide open, and the mantle can be seen through the 

 thick knobby translucent test. 



In Rhodosoma the test is modified so as to form stiff plates recall- 

 ing the valves of a bivalve shell. One plate is attached to the rocks, 

 the other closing against the first like a lid ; the anterior end of the 

 animal with its branchial and atrial orifices is visible only when the 



