50 



The dorsal epidermis in all the species consists, near the middle line, of a specially high columnar 

 epithelium ( v.) which is very conspicuous in C. gracilis (PI. Y, figs. 45 — 52) and in the neuter 

 individuals of C. sidogac (PI. IV, fig. 39). In the latter case, the organ is obviously ciliated, 

 and appears to have a nerve-layer at its base. It is perhaps to be regarded as a sense-organ. 



X. STALK 



The stalk of Cephalodiscns is the muscular aboral part of the metasome. It consists of an 

 epidermis, separated by a well marked basement-meml)rane from a strong mass of longitudinal 

 muscles. The space surrounded by these muscles may be regarded as morphologically part of 

 the third body-cavities, but it is usually filled by connective tissue to such an extent as to leave 

 no definite cavity. The ventral mesentery is, in most cases, indicated by parts which enclose 

 the two longitudinal vessels, but the mesentery does not appear to persist as a complete septum 

 in the adults of any of the species. It has been pointed out by M'Intosh (87, pp. 20, 23) that 

 the truncated base of the stalk is proljabK' used as a sucker. It would be difficult to understand 

 the mode of action of the powerful muscles of the stalk if the animal were not able to attach 

 the base of that organ to the wall of its coenoecium. 



The origin of the stalk from the body differs in the different species. In the buds it is 

 invariably a direct prolongation of the rest of the metasome, as shewn for C. dodccalophus by 

 Masterman (98, 2, Pis. II, III) or in my own figures of the same species (PI. XllI, fig. 181) 

 or of C. gracilis (PI. I, \\>^. 4: PI. Ill, fig. 32). In the adults of C. sibogae (PI. I, fig. 3) this 

 relation is but slightly modified, although even here the stalk originates .somewhat on the anterior 

 or pharyngeal side of the body. In the other species, the origin of the stalk is usually shifted 

 towards the mouth, so that the ventral end of the body projects beyond it, on its posterior 

 side, as a caecum containing the loop of the alimentary canal. But in one specimen of C. gracilis, 

 I have observed a connexion of the stalk with the body precisely like that shewn, for C. sibogae, 

 in fig. 3. The consideration of this case suggests that the appearance of the posterior caecum 

 of the body is caused, to some extent at least, by the contraction of the longitudinal muscles 

 which run from the stalk ])rincipally along the anterior side of the body, and that in the relaxed 

 condition of these muscles the stalk would appear as an almost direct continuation of the 

 body itself. 



In C. dodecaloplms the stalk is usually found directed towards the mouth, as in the well 

 known figure given by M'Intosh (87, PI. II), although the first figure on the next plate of .the 

 same author shews that it can be directed away from the mouth. In young individuals of 

 C. levinseni (PI. I, fig. 5) the stalk is usually in the position which is most commonly found 

 in C. dodecaloplms, lying beneath the edge of the proboscis, as in the specimen figured, or 

 externally to that structure. In old individuals of the same species, I have invariably found it 

 directed away from the mouth (fig. 6), usually describing a short si)iral round the bud or 

 embryos fountl in the proximal part of the zooecium. In C. gracilis and C. sibogae, I have 



