CHORDATA 429 
connected with a plexus of capillaries ramifying in a glandular 
structure, which forms a cap to the anterior end of the noto- 
chord. This is termed the proboscis gland. From the 
posterior end of the collar a well-developed ventral vessel 
passes backward, supported like the dorsal vessel in a median 
mesentery. These chief vessels are placed in communication 
with one another by plexuses of capillaries in the skin and in 
the walls of the alimentary canal, and the skin plexus is 
specialised into a more or less definite circular vessel connect- 
ing the dorsal and ventral vessel in the opercular fold or the 
posterior edge of the collar. The blood is said to be free from 
corpuscles, but the fluid which occupies the remnants of the 
coelom contains amoeboid corpuscles. The course of the 
blood is forward in the dorsal, and backward in the ventral 
vessel. 
The nervous system consists of a dorsal and ventral cord, 
which lie in the skin, and extend from the anus to the posterior 
edge of the collar; at this level the ventral cord divides into 
two strands, which pass round the alimentary canal and join 
the dorsal cord (Fig. 246). The dorsal cord in the region of 
the collar has lost its connection with the epidermis, and by a 
process of delamination, aided by invagination at its ends, has 
come to form a partially tubular cord. This is the portion of 
the nervous system in which the cellular elements are to a great 
extent aggregated. Its posterior end receives the dorsal nerve 
and the two branches of the ventral nerve. In some species 
three nerves arise from this central nervous system and pass 
towards the dorsal skin, these three nerves have been compared 
to the dorsal roots of spinal nerves in Vertebrates. Anteriorly 
this central nervous system is continuous with a well-marked 
sub-epidermic plexus of nerve fibrils which exists in the pro- 
boscis (Figs. 246 and 247); a similar network lies in the skin 
of the trunk, and is continuous with the dorsal and ventral 
nerves. 
The various species of Balanoglossus are all dioecious. 
Both ovaries and testes consist of sacs which open directly on 
to the epidermis, from which they are probably derived. These 
sacs occur in the region of the gill-slits, and open externally to 
the latter; they are also found serially repeated along that 
