296 A. T. MASTERMAN. 



therefore a depression of nervous epiblastic tissue immediately 

 in front of the main nerve-ganglion. It appears to be homo- 

 logous with the "neuropore" of the Chordata in its 

 inceptive stage. The same process of depression involving the 

 uerve-ganglion, and carried further backward, would result in 

 a tubular dorsal nervous system of the same type as that in 

 Balanoglossus and the higher Chordata. 



The three dorsal anterior nerves run forward, and the 

 median one (PI. 19, fig. 6, a. m. n.) continues into a large median 

 nerve-swelling lying immediately under the sense-papilla. 

 Beyond this, however, it can be traced together with the two 

 dorso-lateral nerves (fig. 6, &c., a. I. n.) till they are lost in the 

 pre-oral nerve-ring. This is a thick ring of nervous tissue, 

 found immediately under the pre-oral ciliated band throughout 

 its length. At the posterior corners of the pre-oral hood this 

 ring terminates as such, and appears to branch out in several 

 directions. One branch passes up again to the nerve-ganglion 

 (PI. 18, fig. 2), and numerous fibres also appear to pass on to the 

 ventral surface of the collar region (PI. 19, fig. 10, p. o. n.). 

 The mass of nerve-tracts already referred to as leading out from 

 the '' neuropore," and bending back to this spot, appear also 

 to pass on to the ventral surface of the collar, forming a pair 

 of nervous areas on either side of the mouth. These spread 

 out over the whole ventral surface of the collar as a series of 

 branching fibres. 



Posteriorly the nerve-ganglion passes gradually into a pair 

 of thick nerve-tracts, which run along the dorsal collar region 

 and then diverge. The main mass of fibres passes diagonally 

 downwards immediately behind the tentacles, and, as is seen 

 in sections, exactly at the posterior border of the collar 

 region (PI. 20, fig. 19, c. n. s.). The rest of the fibres pass mid- 

 dorsally as a pair of tracts, giving off branches to the body- 

 wall, and terminating in a nervous ring just anterior to the 

 perianal band. 



The collar-ring behind the tentacles is continued in the 

 mid-ventral line to the same nerve-ring in front of the perianal 

 band, and supplying the ciliated cells of this area with nerves. 



