187 
The notochord is interesting as being particularly long and 
bent. Several polypides were cut into serial sections, and in 
all of them the anterior and posterior ends of the notochord 
appeared more or less sharply bent (text-fig. 3, .a. and n.f.). 
The cavity of the notochord is very distinct ; the thickness of 
the wall of the notochord is least in the basal parts. The apex 
of the notochord is in contact with the thin skeletal layer that 
lies immediately internal to the nerve tract of the dorsal sur- 
face of the buccal shield. Between the dorsal surface of the 
middle part of the notochord and the same sheet of skeletal 
connective tissue is a septum, which separates the right and 
left collar cavities, and is flanked on each side by a sheet of 
muscle, the fibres of which continue down the right and left 
sides of the mouth, and become nearly median again as they 
Sime: 
Ce 

Text-Fic. 4. Section of the pericardial region of the body of Cephalodiscus 
guchristt, cut parallel to the face of the buccal shield. ¢.c.—anterior horn of 
collar cavity ; ec.—tall ectoderm cells of the dorsal wall of the buccal shield, 
cut obliquely ; 4.—cavity of the heart : m.—muscle fibres cut transversely 
and obliquely ; 7.—wall of notochord ; ».c.—cavity of notochord ; 12.4. 
nerve tract ; p.c.—proboscis canal, cut transversely ; pcd.—pericardial Cavity. 
In this diagram, and in text-fig. 3 an attempt has been made to place the 
pen-strokes in such positions and at such distances apart as shall give the 
general appearance and depth of tint of the several tissues as they appear 
in sections examined under the microscope ; the small areas enclosed by the 
pen-strokes are not intended to represent cells, except in a most diagrammatic 
manner, 

attach themselves to the sides of the median septum between 
the right and left collar cavities in the region of the post-oral 
lamella (see the two tracts marked m.s. in text-fig. 3). The 
dorso-anterior wall of the pharynx touches the skeletal laver 
that lies immediately beneath the hinder part of the central 
nerve mass. 
The heart is entirely in advance of the notochord, and in al- 
most all cases is well dilated. The relations of the pericardium 
