58 



individual would have cut the upper Hp before any part of the diverticulum was reached, and 

 in the other, the first part to appear in the sections would have been the caecal end of the 

 diverticukmi (as in Masterman's figs. 90 — 92). The arrangement of the oral or pharyngeal 

 grooves seen in the sections would thus have been very different in the two cases. Figs. ^y 

 and 43, of C. gracilis, illustrate the same point. On the other hand it is clear that in the case 

 of an organ whose walls are so crumpled as is the pharynx in fig. 42, considerable care is 

 necessary to distinguish the permanent folds from those W'hich have no dehnite significance. 



In describing my own observations I begin with the consideration of a frontal series of 

 sections of a comparatively young blastozooid of C. dodccalopJms, in which the structure appears 

 to be but little complicated by secondary folding. The first section figured (PI. XII, fig. 152) is 

 dorsal to the mouth, and passes through the external aperture of the right oviduct, the dorsal 

 diverticulum of the pharynx {div}^ and the notochord [nc/i.). All the body-cavities are visible, 

 the third cavity being subdivided on each side by the ovarian mesentery {ov. ;;/.). On the left 

 side, the section passes through the arm-base, just dorsally to the beginning of the operculum. 

 On the right side, the lateral lobe of the operculum {op. r.) is cut in the main parallel to its 

 flat surfaces; and the tentac.les of the third and fourth arms are visible. The dorsal diverticulum 

 of the pharynx is circular in section, its cavity being markedly triradiate. One of the grooves 

 of the cavity is median and posterior, though shewing indications of being composed of 

 Masterman's two "dorsal grooves", the other two are paired and on the anterior side. Each of 

 the three ridges which separate the grooves from one another is marked by a deeply stained 

 patch of cells which appear to be glandular. In more dorsally situated sections the glandular 

 area is continuous round the anterior side of the diverticulum -, while more ventrally, patches 

 which stain in the same way and seem to be unicellular glands, appear here and there on the 

 lower lip, at the sides of the mouth, and even in the oral epidermis of the operculum. 



In the next section but one (not figured) the base of the notochord is cut longitudinally 

 (cf. PI. IV', fig. 42), its lumen being traceable into the outer part of the anterior ridge of the 

 pharyngeal diverticulum. 



In fig. 133 (two sections later) the left lateral lobe of the operculum {op./.) is cut 

 horizontally, a fold (ƒ.) of its body-wall marking off the section of the collar-cavity which 

 contains the collar-canal from the more externally placed operculum itself. The section passes 

 just ventrally to the base of the notochord, which is of course not seen. The pharyngeal 

 diverticulum has much the same structure as in the former figure, except for the fact that the 

 dorsal walls of a pair of lateral outgrowths of its cavity are involved by the section. The 

 posterior ends of these walls are composed of a tissue which stains but slightly, and they 

 constitute the beginning of what Masterman (97, 2, p. 353, note) has described as the "pleuro- 

 chords". I make use of this term for descriptive purposes without in any way accepting the 

 homology indicated by Masterman in using it. 



Somewhat further ventrally (fig. 154) the lateral outgrowths open into the median part 

 of the diverticulum, the pleurochordal tissue in their walls being sharply marked. The three 

 grooves seen in the lumen of the median part are present as before. 



F'ö- 155 cuts the dorsal edge of the mouth; and the left anterior groove is seen to 



