REGIONS OF THE BODY. SKIN. 



09 



, 



slightly over the anterior end of the trunk, so that there is a 

 circular groove at the junction of the collar and trunk. In 

 some species this hind end of the collar may project so far back 

 as to cover three or four gill- slits (Dolichoglossus koivalevskii), 

 and in this case the projection has been called by Bateson the 

 atrial fold, and the space 

 which it encloses the atrial 

 cavity (cf. Ampliioxus). 



The trunk itself is also 

 indistinctly divided into re- 

 gions : these are the bran- 

 chiogenital immediately 

 succeeding the collar (Fig. 

 56, k, g) ; next, the hepatic 

 (vb), in which the intestinal 

 walls contain a green or 

 brown pigment ; and lastly 

 the abdominal region which 

 forms the hind end of the 

 body. 



The branchiogenital re- 

 gion possesses in its anterior 

 part on each side of the 

 dorsal middle line a double 

 row of pores ; these are the 

 branchial pores which place 

 the gill-apertures of the 

 alimentary canal in com- 

 munication with the ex- 

 terior. The branchial pores 

 are placed in a slight 



groove, the branchial groove. Posteriorly the branchiogenital 

 region is without branchial apertures. The genital organs are 

 also found in. the lateral walls of the body in this region. 



The skin consists of a layer of ciliated epidermis or ectoderm 

 beneath which is a structureless basement membrane. The 

 epidermis is in most parts a thick layer and appears to consist 

 of very long and narrow cells, extending indeed through the 

 whole thickness (Fig. 57) with interspersed unicellular glands. 

 Moreover the nuclei occur at different levels, thus suggesting at 





FIG. 56. Dolichoglossus kowalevskii (after A. Agas- 

 siz, from Korschelt and Heider). e proboscis ; 

 kr collar ; k anterior part of branchial region ; 

 g genital region ; db dorsal, vb ventral middle 

 line. 



