Introduction to Animal Morphology. 231 



CHAPTER XXXIII. 



CLASS 13. TUNICATA. 



SINGLE or aggregate, fixed or free, hermaphrodite,* 

 marine Molluscoids ; rarely symmetrical ;f developed 

 with metamorphosis ; varying from the size of a pin's 

 head to that of an apple, or larger in colonial forms ; 

 and sometimes divisible into three parts (named thorax, 

 abdomen, and post-abdomen). The epidermis is often 

 incrusted with sand, &c., or uneven, and sometimes 

 consists of two layers, a superficial, non-cellular in 

 Doliolum and Appendicularia, becoming cellular in 

 others by immigration of cells, and a deeper epithelial 

 containing larger cells compound to those of the 

 chorda dorsalis of vertebrates, and crystals of calcium 

 carbonate, and often stellate (Botryllus, &c.) or glo- 

 bular (Didemnium) spicules also of lime, rarely sili- 

 ceous (Salpae). Similar calcareous bodies in the flat 

 Cholyosoma form two circlets of eight plates of horny 

 consistence, four around the branchial, three around 

 the atrial opening, and one intermediate. In Appen- 

 dicularia the cuticle forms a remarkable case (the 

 Haus, of ^fcrf^ns} y said to exist only in males.* 



The cutis is never separate from the cuticle, and forms 

 with it the sn-rallecl ouk-r wall of the mantle ; it con>: 

 connective tissue and pigment corpusrlrs (often stellate), and 



i a copious intercellular matrix, which < 

 Tunicine, Ci t H w Oio, only differing from Cellulose by 



Doliolum is 



'.ly symmetrical. 



J If Stlii/ascus be a true tunica 1 i valve 



shell (Lmaze Duthiers). 



