34 



W. G. RIDEWOOD. 



of four or five polypides. An attempt has been made to place the pen -strokes and 

 dots in such positions and at such distances apart as shall give the general appearance 

 and depth of tint of the several tissues as seen under the low power of the microscope ; 

 the small rectangular and polygonal areas enclosed by the pen lines are not intended 

 to represent cells, except in a most diagrammatic manner. 



The general internal structure of Cephalodiscus nigrescens does not differ in any 

 important respects from that of C. dodecalophus as made known to us by the writings 

 of Harmer and Masterman. By its superior size C. nigrescens presents great advan- 

 tages over C. dodecalophus as an object for investigation, but, while most of the 

 details of the chordal, nervous, muscular and coelomic systems described in accounts 

 of the latter species can be recognised in the former, only a certain proportion 

 of the blood-vessels described by Masterman in C. dodecalophus can be identified 



pc.d. h. 



TEXT-FIGURE 10. Median longitudinal section of the antero-ventral part of a polypide of Cephalodiscus nigrescens. 



b.s. = thickened ventral wall of the buccal shield ; c.n.m. = central nerve mass ; e.p. = ectodermal pit ; 

 go. = gonad ; h. = heart ; m. = mouth ; mu.\a, mu.lc, mu.2 = muscle (see text) ; no. = notochord ; p.c. = cavity 

 of the buccal shield, proboscis cavity; pcd. = pericardium; ph.d. = pharyngeal diverticulum ; po.l. = posterior 

 part of the post-oral lamella ; r .1. = red line of the shield ; i.e. = trunk cavity. 



with any degree of confidence in C. nigrescens, and no light is thrown on the course 

 taken by the blood in the vessels. 



The notochord (subneural gland of Masterman) has a well-defined and continuous 

 lumen, and does not open into the dorso-anterior diverticulum of the pharynx, as, 

 according to Harmer (10, p. 54) it does in C. dodecalophus ; its opening is situated 

 more ventrally (text-fig. 10, no.). The anterior end of the notochord is in contact 

 with the central nerve mass, as also are the pericardium and the pharyngeal diverti- 

 culum (ph. d.). The pericardial sac does not extend over the dorsal side of the tip 

 of the notochord as it is described as doing in C. dodecalophus, but it projects back 

 beneath the notochord to a greater extent than in the latter species. The heart 

 is not fixed to the end of the notochord, but to the ventral side of its extremity. 



Between the notochord, the central nerve mass, and the pharyngeal diverticulum 



