8 CHORDATE ANATOMY 



phosis. The sexually-produced tailed larva bears certain striking resem- 

 blances to the larva of Amphioxus. 



Some systematists recognize 1400 species. 



ClONA is a sessile tunicate, three or four inches in length, which is 

 attached by tunicin stolons to its substratum. A tunicin test or tunic, 

 which is secreted by the skin, encloses the entire animal as a sac. Beneath 

 the test and loosely connected with it, except in the region of the two 

 apertures of the body, lies the body-wall or mantle. This consists of an 

 external simple epithelial ectoderm, and, beneath this, connective tissue 

 containing a network of muscle fibers which are more abundant in the 



BRAIN 

 CILIATED FVJNNEC 

 MOUTH 

 GILL SLITS 



■ATRIAL CAVITY/ SPINAL CORD 



\notochord 

 intestine 



\ENDOsrYLE "^"^ B. METAMORPHOSIS. 



Fig. 9. — Diagrams of stages in the metamorphosis of a uruchordate larva. When 

 the larva settles down and becomes fixed by its adhesive papillae, the tail is lost and the 

 notochord disappears. Thus the chordate characters which are so evident in the larva 

 are partly lost in the mature organism. (Redrawn from Korscheldt and Heider, after 

 Seeliger.) 



region of the two apertures of the body, which they serve to close and 

 open. 



Of the two external apertures, the more ventral is the inhalent or 

 oral siphon and the other the exhalent or atrial siphon. The former leads 

 directly to the mouth, which is surrounded by a circle of tentacles. The 

 mouth leads into a greatly enlarged pharynx, which is perforated by 

 numerous gill-slits or stigmata. The action of the cilia on the bars of 

 these slits serves to maintain a current of water from the pharynx into 

 the surrounding peribranchial or atrial cavity. Such relations resemble 

 those of similar organs in Amphioxus. In the floor of the pharynx extends 

 a longitudinal groove, the endostyle, which morphologists generally 

 homologize with the thyroid gland of vertebrates. A somewhat similar 

 groove extends also along the dorsal side of the pharynx. The alimentary 



