316 



METAZOAN PHYLA 



front of the collar. There is a dorsal pouch at the anterior end of the 

 alimentary canal which originates like a notochord as a dorsal outpocket- 

 ing of the archenteron and which runs forward into the proboscis. Its 



^Apical sense organ 

 Ciliated band 



Heairi- 



Mouth 



Pore entering 

 proboscis cavity 



^Anal ciliated 

 ring 



Oral funnel 



-Atrial 

 funnel 



''Anus 



Fig. 216. — Tornaria larva, viewed from the side. (From Thomson, "Outlines of Zoology," 



after Spengel.) 



function is to stiffen the proboscis, which is an aid in burrowing. The 

 claims of Balanoglossida to inclusion in the phylum Chordata rest upon 



the fact that this structure is consid- 

 ered a notochord, that the animals 

 possess a large number of pharyngeal 

 slits, and that in the region of the collar 

 a portion of the dorsal nerve cord seems 

 to be definitely tubular. The dorsal 

 nerve cord is also formed by a process 

 of invagination. This type is mone- 

 cious. The larva, called a tornaria 

 (Fig. 216), is free-swimming and resem- 

 bles the larvae of the echinoderms. 



340. Urochordata.— This subphylum 

 includes the tunicates, which may be 

 either free-swimming or fixed forms. 

 The typical tunicates, also called ascidi- 

 ans, are fixed forms in the adult stage 

 (Fig. 217) and are inclosed in a tunic 

 composed of animal cellulose the com- 

 position of which is very similar to that 

 of plant cellulose. It is the presence 

 of this tunic which suggests the name 

 tunicate. When one of these animals is stimulated, the body is strongly 

 contracted and the water is thrown out through the mouth with consider- 



FiQ. 217. — Pyura aurantium 

 (Pallas), a sessile ascidian from 

 Labrador. X %. {From VanName, 

 Proc. Boston Soc. Nat. Hist., vol. 34.) 



