TUNICATA. 93 
observe the tadpoles become fixed and develop into fixed Ascidians. 
He calls the active little swimming larvae "Spinulse," 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 mentula, described above, belongs to this family. 
Chelyosoma is characterised by the test forming tortoise-like 
horny plates on the upper surface. The exhibited specimen of 
Fig. 12. 
/ ? ^~v. 
«i 'A- ------ " #^ 
Chelyosoma macleayanum, slightly enlarged. 
a, branchial ; b, atrial orifice. 
C. 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 
