298 A. T. MASTERMAN. 



are seen, and in addition the ventral nerve-tract is seen to 

 be novr clearly separated into two ventro-lateral areas [v.c.n.) 

 with columnar ciliated cells, though the nerve-fibres may be 

 also traced somewhat prominently in the mid-ventral line. In 

 PI. 21, fig. 39, the trunk-area alone is seen in section, and 

 ventral nervous areas can be just discerned, though their 

 demonstration is easier in surface views. Thus the nervous 

 system of Actinotrocha consists of — 



1. A central ganglion lying in the front collar region and 

 between this and the pre-oral lobe, with the epiblast imme- 

 diately in front, depressed to form a neuropore. 



2. A ring round the posterior part of the collar, continued 

 from the ganglion dorsally and ventrally, giving off fine double 

 groups of nerve-tracts to the anal end of the body. 



3. Groups of fine nerve-tracts continued dorsally along the 

 trunk from the hind end of the collar to the anal end of 

 the body. 



4. A ring around the anal end of the trunk, into which the 

 dorsal and ventral tracts lead. 



5. A ring round the edge of the pre-oral lobe, joined at 

 each side to the ganglion, and in the median front region by 

 three main tracts running in the mid-dorsal line forwards from 

 the ganglion. 



6. A diffuse plexus of fibres at the base of nearly all the 

 epiblastic layer, conspicuous amongst which are the fibres of 

 the ventral collar area, which pass forwards and dorsally to 

 meet the ganglion. 



A comparison of this nervous system with that of Balano- 

 glossus and Cephalodiscus will be instituted later. 



Sense-organs. — At a stage later than fig. 1 (vide fig. 2) 

 the mid-dorsal surface of the pre-oral lobe is elevated into a 

 cone-shaped process, which has slightly columnar cells, im- 

 mediately under which is a swelling of nervous fibres pro- 

 truding into the cavity of the pre-oral lobe (fig. 20, s.p.). The 

 examination of fresh specimens is required for determination of 

 the further structure of this organ. 



Pre-oral Ciliated Ring. — Round the edge of the pre- 



