1O 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, d.n, v.n), a circular tract 
(Fig. 3, ¢.m) connecting these two at the posterior edge of the 
colar, 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, ns), runs 
through the cavity 
of the collar, but is 
connected with the 
epidermis at each 
end. It thus be- 
Vv. comes continuous 
Fic. 4.—Ptychodera bahamensis, Bahama Is, Transverse in front with the 
section through the branchial region. 6, Branchial 
part of pharynx ; 0.c®, third body-cavity ; d.m, dorsal nerve-layer on the 
mesentery ; d.n, dorsal nerve; d.v, dorsal vessel ; e, es Sar rates bs 
epidermis, with nerve-layer (black) at its base ; g, genital 1 oboscis ; stalk, 
wing ; g.p, gill-pore, encroached on by the tongue-bar While posteriorly 1t 
(t); ¢, lateral septum; m, longitudinal muscles; 0, . . 
oesophageal or alimentary part of pharynx ; 7, repro- passes into the dor- 
ductive organ ; ¢, tongue-bar ; v, ventral mesentery and gal 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 “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 Ptychodera flava, 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 
