lO 



HEMICHORDATA 



CHAP. 



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, cl.n, v.n), 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 

 tlirough the cavity 

 of the collar, but is 

 connected with the 

 epidermis at each 

 end. It thus be- 

 "^-v.Ti, comes continuous 



Fig. i. — Ptychodera hahamensis, Bahama Is. Transverse jj^ front with the 

 section through the branchial region. J, Branchial 



part of pharynx ; b.c\ third body-cavity; d.m, dorsal ncrve-laysr On the 



mesentery; d.n^ dorsal nerve; d.i\ dorsal vessel; e, i-irobrmpiq ^talk 



epidermis, with nerve-layer (black) at its base ;*/, genital ^ ' ' 



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



(t); I, lateral septum; m, longitudinal muscles; o, „„„„„„ 4„4.. ii-,„ J„„ 



oesophageal or alimentary part of pharynx ; r, repro- pahbCb J-nuu me UOI- 



ductive organ ; t, tongne-bar ; o, ventral mesentery and sal and the circular 

 ventral vessel ; v.n^ ventral nerve. {After Spengel. ) , , -r 



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 "neuropore" (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 Glossobalanus sarniensis and Ptyclwdera fiava, 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 



