Introduction to Afizmal Morphology. 23 1 



CHAPTER XXXIII. 



CLASS 13. — TUNICATA. 



Single or aggregate, fixed or fi-ee, hermaphrodite,* 

 marine Molluscoids ; rarely symmetrical ;t 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 

 Chelyosoma 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, oi Mertens)^ said to exist only in males. :J: 



The cutis is never separate from the cuticle, and forms 

 with it the so-called outer wall of the mantle ; it consists of 

 connective tissue and pigment corpuscles (often stellate), and 

 crystals, in a copious intercellular matrix, which consists of 

 Tunicine, CijHioOio, only differing from Cellulose by being 



* Doliolum is dioecious, 

 t Pelonaia is bilaterally symmetrical. 



\ If Schizascus be a true tunicate, it is remarkable as having a bivalve 

 shell {Lacaze Duthiers). 



