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 
cob Gi 
Il | ph.c. 
Wb. 
--mu.2 
i PO at 
Trxt-Ficure 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; muw.la, mu.1c, mu.2 = muscle (see text); no. = notochord; p.c. = cavity 
of the buccal shield, proboscis cavity; ped. = pericardium; ph.d. = pharyngeal diverticulum; po.l. = posterior 
part of the post-oral lamella; 7.1. = red line of the shield; t.c. = 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 
