442 



CHORDATA 



were grouped with the worms, Init their de\elopment shows them to be 

 more nearlv related to the \ertehrates. 



The name is due to the tunic or mantle — lacking in the Copehxt:v — 

 an envelope (tig. 400, /) formed, like a cuticle, Iiy 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 

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

 an interesting chemical nature. It has the same chemical composition 



Ftg. 406. — Diagram of a lunicalo (orig.l. a. atrium; h. nervous gangiion: c, cndo- 

 stylc; /. iiik-sline; iii, moutli; ii, subneural glaiul; s, slonwcli; /, tunic. ' In tlic centre tlie 

 branchial liasl<et witli the gill slits communicating; «illi the |iei-il)rancliial space, amt 

 this in turn with the atrium. 



(C,;HinO.) as cellulose and agrees with this substance, so characteristic 

 of plants, in its reactions. No other animals ha\e so much cellulose. 



The anterior part of the digesti\e tract is modified into a pharynx 

 or liranchial chamber, the walls of which arc perforated with a varvin" 

 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 alriiim 

 (a), l)efore reaching the outsiile world, ^\■hile the respiratory water 

 passes through the gill slits the food i)articlcs which it contains are re- 



