ACRANIA 287 



625. Class Leptocardia. There is only one class represented 

 by a few species. The form of the body has given rise to 

 the common name "lancelet." The integument of the lancelet 

 consists of a single layered epidermis. The gill slits open into 

 a peribranchial chamber formed by folds of the skin. This 

 chamber opens to the exterior by a pore at its posterior end. 

 The mouth is surrounded by a circle of cirri. The animal lies 

 on its side partly buried in the sand and collects its food from 

 the respiratory current which is produced by ciliary action. 

 The intestine is a straight tube opening by a vent on the left 

 side of the tail. A long glandular pocket connected with the 

 intestine probably represents a digestive gland. There is no 

 heart but the larger vessels drive the blood by pulsating 

 contraction. There is a ventral vessel (truncus arteriosus) which 

 carries the blood forward. From this lateral branches pass 

 over the gill arches to unite above in another vessel through 

 which the blood flows back toward the posterior end of the body. 

 A portal vein connects the intestine and digestive gland (liver), 

 and other veins from the body-wall and digestive gland unite 

 in the truncus arteriosus. In the region of the gills there is a 

 paired series of nephridia which begin with funnels in the 

 ccelomic cavity and open into the peribranchial chamber. 

 The lancelet cannot be said to have a brain. The anterior 

 end of the nerve tube (spinal cord) is slightly enlarged and con- 

 tains a vesicular enlargement of the central canal. Connected 

 with this is a single eyespot and an olfactory groove. Numer- 

 ous other eyespots lie scattered along the spinal cord. The 

 gonads lie against the wall of the peribranchial chamber which 

 breaks to permit the reproductive cells to escape. The develop- 

 ment of the lancelet is by a metamorphosis. The early stages 

 are described in Part III. 



626. PHYLUM XII. Vertebrata. The Vertebrates all have a 

 series of gill slits. In the terrestrial forms these are only 

 present in the larval stages. The nervous system arises as a 



