20 CTENOPHORA. 



The shape of the embryos is now rather different from that of tlie previous stages; they are 

 distinctly elongate in the transversal plane, as seen in Figs. 8 — 9, PL II, and towards the oral 

 side they are narrowed, being tluis pear-shaped when seen from the transverse plane (PI. II, Fig. io|. 

 Contemporaneonsly they have increased somewhat in size, being now ca. i'5"'"' long (transverse diameter) 

 b\- ca. 1-2"'"' broad (sagittal diameter). The now fully developed costse are sunk down in deep lodges, 

 the four radial parts of tlie body arching out between the cost^e and partly covering them, so that 

 the combs protrude from four deepenings. The four radial archings also rise over the apical organ, 

 which is rather deeply sunk (PI. II, Figs. 8 — 9). 



The transverse furrow is now very deep and reaches so far upwards as to iuchide the opening 

 of the tentacle sheath (PI. II, Fig. 10). Tlie lobes may be pressed together or more ©r less opened, even 

 so widely that the basal surface comes to be quite flat (PI. Ill, Fig. 6). (The same may be observed 

 in the young of EucJiaris. Comp. Chun. Monogr. p. 128. PI. IV, Fig. 10). When the lobes are thus 

 opened, the mouth-opening is seen in the bottom of the furrow as a transverse slit (viz. transverse 

 to the furrow). (PI. II, Fig. 11). — The larva in this stage agrees in several important respects with the 

 larval form of the Lobatae, especialh* that of Eucharis iimlticoniis as described by Chun (Monograph, 

 p. 122. PI. IX, especially figs, i — 2). 



The apical organ has now reached its full development. It is deeply sunk (PI. II, Fig. 10, 

 PI. Ill, P'ig. I, PI. VIII, Fig. 13, PI. X, Fig. 9), a narrow channel limited by the four aboral elevations 

 of the body leading down to it (PI. Ill, Fig. i). The sections figured in PI. VII, Figs. 3 and 10 give 

 some information on its finer structure (to compare with PI. VII, Fig. 6 representing its structure in 

 the foregoing stage). The main difference from the foregoing stage is that there has now been formed 

 a complete cupule over the otolith, which latter has been considerably augmented. The zone of cells 

 forming the cupule is still quite distinct. Whether the cells in the bottom of the depression are 

 ciliated in this stage I have not been able to see quite distinctly; but I can scarcely doubt that the 

 zone between the nuclei of the epithelium and the otolith shown in PI. VII, Fig. 10 partly, at least, 

 consists of a mass of cilia corresponding to that seen iu the figure of the organ in the foregoing stage 

 (PI. VII, Fig. 6) ; in a pair of the sections I think I can discern in some places a limit between the 

 cilia and the epithelium, as it is represented in PI. VII, fig. 6. Any trace of balancers could not be 

 detected. Seen from above (PI. X, Fig. 11) the walls of the organ are folded inwards in the transversal 

 axis, while in the sagittal axis there is seen an apparently kidney-shaped body to each side of the 

 organ (PI. X, Fig. 11, p. f.). These bodies represent the polar fields, which lie wholly within the 

 depression, though above the apical organ itself. On preparing the whole apical organ out the real shape of 

 the polar field is seen to be nearly that of a horseshoe (PL X, Fig. 10). Of its finer histological structure 

 no satisfactory information can be given; sections through it are shown in PL VII, Figs. 7 and 8. — 

 As stated above (p. 7) I have been unable to find any traces of the polar fields in the grown animal; 

 it thus appears that these organs degenerate later on; also the cupule and the cells forming the cupule 

 appear to have degenerated in the grown specimens. 



In PL III, Fig. 2 are represented some fine lines radiating from the apical organ to the upper 

 end of the costse. There can certainly be no doubt that they correspond to the ciliated epithelial 

 strands connecting the apical organ with the cost?e in other Ctenophorcs, regarded by Chun as 



