10 



HE.MICPIORDATA 



dv 



d.n 



between the epidermis and the nervous system is usually restricted 

 to embryonic life in other animals. 



The main nerves of Balanoglossus are a dorsal and a ventral 

 tract in the trunk region (Fig. 4, d.n, v.ri), a circular tract 

 (Fig. 3, c.n) connecting these two at the posterior edge of the 

 collar, and a strong concentration of nerve-tissue round the whole 

 of the proboscis-stalk, and of the posterior end of the proboscis 

 (Fig. 3). In the region of the collar the nervous system attains 



its highest develop- 

 ment, taking the 

 form of a median 

 cord passing above 

 the alimentary 



canal. This cord, 

 known as the cen- 

 tral nervous system 

 (Fig. 7, n.s\ runs 

 through the cavity 

 of the collar, but is 

 connected with the 

 epidermis at each 

 end. It thus be- 



l 



m. 



comes continuous 

 in front with the 



FIG. 4. Ptychodera bahamensis, Bahama Is. Transverse 

 section through the branchial region. b, Branchial 

 part of pharynx ; b.c 3 , third body-cavity ; d.m, dorsal nerve-layer On the 

 mesentery; d.n, dorsal nerve; d.v, dorsal vessel; e, 1 - 1T . n V ino ] ofolV 



epidermis, with nerve-layer (black ) at its base ; g, genital r 1 1 K > 



wing ; g.p, gill-pore, encroached on by the tongue-bar while posteriorly it 

 (t) ; I, lateral septum ; m, longitudinal muscles ; o, + -1 -\ , 



oesophageal or alimentary part of pharynx ; r, repro- P aSf 



ductive organ ; t, tongue-bar ; v, ventral mesentery and sal and the circular 

 ventral vessel ; v.n, ventral nerve. (After SpengeL) 



nerve - tracts. In 



nearly all cases the epidermis is pushed into the cord at the 

 points where it passes into the skin, in the form of an anterior 

 and a posterior "iieuropore" (Fig. 3). A transverse section 

 through the extreme front or hind end of the collar accord- 

 ingly shows a tubular nervous system. In certain species, 

 as in Glossolalanus sarniensis and Ptychodera Jlava, a central 

 canal, opening in front and behind, exists throughout the entire 

 length of the central nervous system, while in G. minutus a 

 canal of this kind occurs in the young animal, but not in the 

 adult. The central nervous system is developed as a longitudinal 



