EARLY STAGES IN DEVELOPMENT OP BALANOGLOSSUS. 151 



up of four parts : a dorsal and ventral cord lying in the skin in 

 the dorsal and middle lines respectively, connected with each 

 other by a perioesophageal ring in the posterior fold of the 

 collar ; a continuation of the dorsal nerve through the collar, 

 first as a solid cord, and afterwards as a hollow tube running 

 in the body, separated from the skin by mesoblastic tissues, 

 this collar portion being continuous with the skin at both ends ; 

 a continuation of the dorsal cord on to the proboscis, at the 

 base of which it forms a considerable concentration, forming a 

 ring in connection with the skin round the base of the pro- 

 boscis stalk; lastly, a plexus of nerve-fibres over the whole of 

 the body in close connection with the skin, which is particu- 

 larly well developed on the proboscis. 



2. The Proboscis-pore is a small ciliated opening into 

 the body cavity of the proboscis, situated in the middle dorsal 

 line in the region of the proboscis stalk. It opens through 

 the thickest part of that concentration of the nervous system 

 which encircles this part of the body (Spengel (7), &c.). 



As will be afterwards shown, when the anatomy of the older 

 stages is treated of, this pore leads into a small chamber lined 

 with ciliated columnar cells. This chamber is continuous with 

 that horn of the anterior body cavity which lay originally on 

 the left side of the gut. It thus communicates with the general 

 tissue spaces of the proboscis. The right posterior horn never 

 becomes connected with the exterior. 



3. In the adult a forwardly-directed diverticulum opens into 

 the alimentary canal in the dorsal middle line, a little behind 

 the mouth. For the cliief part of its course it lies in the 

 proboscis (Spengel). 



It consists of large vacuolated cells, whose structure is some- 

 what peculiar, and bears a strong resemblance to the noto- 

 chordal tissue of a young Elasmobranch. 



The lumen of this diverticulum is large posteriorly, and 

 anteriorly is almost entirely obliterated. This rod of tissue is 

 the supporting structure of the proboscis. 



It arises as a forward growth of the hypoblast at Stage G, in 

 a manner to be afterwards described. 



