/. LEPTOCARDIL 



503 



primitive eye, but other places are sensitive to light. The olfac- 

 tory organ is an unpaired pit on the anterior end of the body; 

 and at its bottom, in the young, is an opening, the anterior neu- 

 ropore, which leads into the anterior end of the neural canal. It 

 is a point of delayed closure of the embryonic medullary folds. 



Of the alimentary tract more than a third is occupied by the 

 pharynx with the gill slits. It begins with an oval mouth, sur- 

 rounded by cirri, and is perforated by numerous gill slits, be- 

 tween which elastic gill arches form a support for the walls (fig. 

 542, kb). In the young the gill slits open directly to the anterior, 

 but later, somewhat as in Tunicata, into a peribranchial chamber 



FIG. 542. Section of the gill region of Amphioms. (After Lankester and Boveri.) 

 a, aorta descendens; 6, peribranchial space ; r, notochord ; co, ccelom (branchial 

 body cavity) ; e, hypobranchial groove, beneath it the aorta ascendens ; g, gonad; 

 kb, gill arches: fcd, pharynx; I, liver ; m, muscles; n, nephridia, on the left with 

 an arrow; ?, spinal cord ; *n, spinal nerve ; sp, gill slit. 



{#) which allows the escape of the water through a porus branchialis 

 (fig. 541, JP), behind the middle of the body. On the ventral floor 

 of the pharynx is a ciliated hypobranchial groove (fig. 542, e), the 

 homologue of the tunicate endostyle and of part of the vertebrate 

 thyroid. It leads back to the straight digestive tract which opens 

 on the left side near the end of the body, and bears in front a 

 blind liver sac which extends forward into the gill region (figs. 



