/. LEPTOCAIiDII. 



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, tlie 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 tiian 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 supjiort for the walls (fig. 

 54;2, Jdi). In the young the gill slits open directly to the anterior, 

 but later, somewhat as in Tunicata, into a peril^ranchial chamber 



Fig. 54:?. — Section of the gill region of Amphioxus. (After Lankester and Bnvpn.) 

 o, aorta ilesoendens; h, peribranchial space ; r, notochorJ ; cb, coelom (branrhial 

 bodv favitv) ; e, hypobrancliial uroove, beneath it the aorta ascendens ; ii, u'onad ; 

 ih, gill arches; kd, pharynx; (, liver ; m, muscles; ii, nephridia, on the leU with 

 an arrow; i, spinal cord ; s», spinal nerve ; sp, gill slit. 



{[)) which allows the escape of the water tlirough a poms branchialis 

 (fig. 5-fl, p), behind the middle of the body. On tlie ventral floor 

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

 homologue of the tunicate endostylo and of part of the vertebrate 

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

 on the left side near the end of the l)ody, and bears in frf)nt a 

 Idind liver sac whicli extends forward into the gill region (figs. 



