CHORD AT A 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 

 speciaHsed 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 coUar 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 spiual 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 



