52 



which is attached to the oesophagus and the part which bears the anterior limb of the vascular 

 loop of the stalk. In fig. 127 the epidermis is raised up into a projection, inside which the 

 musculature of the body-wall is taking on the arrangement characteristic of the stalk. The 

 ventral mesentery passes from the stomach to the limiting layer of the connective tissue of the 

 stalk, with which it is specially connected on the left side of the figure. The triangular space 

 of fig. 126 is no longer distinguishable from the other spaces in the feebly developed connective 

 tissue of the stalk. The muscles, in this and in other sections, have a tendency to arrange 

 themselves in conuli, the apices of which project towards the centre of the stalk. 



Fig. 128 cuts the stalk immediately before it separates completely from the body; and it 

 thus shews the posterior limb (/. :>.) of the stalk-vessel at the point where it enters the body. 

 The anterior limb {a. v.) of the same vessel is still indicated by a thickening of this part of the 

 mesentery. Fig. 132, which has already been described, is a section, somewhat more highly 

 magnified, of the stalk of the same individual, through a part which has become completely 

 separated from the body-wall. 



It may be noted here that the stalk of Rliabdopleiira has a structure essentially cor- 

 responding with that of Cephalodiscus. This follows from the recent accounts given by Schepotieff 

 (04, p. 10) and Fowlf.r (04, p. 24). The stalk of Rhabdopleiira, is, however divided by a 

 complete median mesentery. According to Fowler's description and figure (PI. III, fig. 2) a 

 nerve-tract, triangular in transverse section, is found on the "ventral" side of the stalk, e.xactly 

 as in C. levinseni. In the immediate neigbourhood of this is a space (^), regarded by Fowler 

 as perhaps artificial; while next to this, and almost in the middle of the mesentery, is another 

 space marked by Fowler e7id.} This cavity is "continuous with the lining of the alimentary 

 canal" (p. 29), and Fowler rejects the view that it is a blood-vessel, suggesting that it is an 

 endodermic structure, which is concerned in the formation of the alimentary canal of the buds. 

 This view is further alluded to in Section XVI, but in the mean time I may point out that 

 the two cavities described by Fowler in the stalk-mesentery of Rhabdopleitra have precisely 

 the same arrangement as the two parts of the vascular loop which e.xtends into the stalk of 

 Cephalodisc7is. 



The stalk of C. gracilis (fig. 134), differs from that of either of the species which have 

 already been described. The epidermis of the anterior side forms a large ridge projecting towards 

 the centre. This is usually bisected by a vertical line of pigment, on either side of which is a 

 layer of nerve-fibrils. The ridge is relatively more prominent than in C. levinseni, and may be 

 parallel-sided in transverse section. The muscular layer is reflected round this ridge; and while 

 it thus has fundamentally the same arrangement as in C. levinseni, the shape of the muscular 

 mass is somewhat different. Connective tissue is hardly developed in the stalk, which thus 

 contains a definite cavity. The vessels of the stalk, instead of appearing as minute slits in the 

 remnants of the mesentery, are commonly very conspicuous in transverse section, and appear 

 as large cavities with a delicate wall. In some cases (fig. 134) they are obviously anterior and 

 posterior in position, but they often lie side by side in the body-cavity of the stalk (fig. 135), 

 although it can be ascertained, in some of these cases, that one of the vessels is closely 

 related to the nerve-ridge of the anterior side of the stalk, while the other is suspended by an 



