442 



CHORD ATA 



were grouped with the worms, but their development shows them to be 

 more nearly related to the vertebrates. 



The name is due to the tunic or mantle lacking in the Copelatae 

 an envelope (fig. 496, /) formed, like a cuticle, by the epithelium of the skin, 

 but distinguished from ordinary cuticula by its structure. It resembles 

 connective tissue in that cells from the mesoderm wander into the ground 

 substance, which is sometimes fibrous, sometimes homogeneous, and has 

 an interesting chemical nature. It has the same chemical composition 



FIG. 496. Diagram of a tunicate (orig.). a, atr'.um; />, nervous ganglion; e, endo- 

 style; /, intestine; m, mouth; H, subneural gland; s, stomach; /, tunic. In the centre the 

 branchial basket \vith the gill slits communicating with the peribranchial space, and 

 this in turn with the atrium. 



(C 6 H 10 O 3 ) as cellulose and agrees with this substance, so characteristic 

 of plants, in its reactions. No other animals have so much cellulose. 



The anterior part of the digestive tract is modified into a pharynx 

 or branchial chamber, the walls of which are perforated with a varying 

 number of gill slits, these leading either directly to the exterior or, more 

 usually, into a peribranchial chamber, and from this to a cloaca or atrium 

 (a), before reaching the outside world. While the respiratory water 

 passes through the gill slits the food particles which it contains are re- 



