18 W. G. RIDEWOOD. 



The alimentary canal of Rhabdopleura resembles that of Cephalodiscus in the 

 position of the mouth and anus, the flexure of the gut, and the presence of pleurochorcls 

 and notochord ; even the gastric caecum of Cephalodiscus is represented (" Blinddarm " 

 of Schepotieff, 1904, p. 14). 



The notochord (Fowler and Schepotieff) is an anteriorly directed diverticulum 

 of the first part of the alimentary canal. It is usually solid, but in some polypides 

 possesses a cavity. The cells of the hinder or basal part are frequently vacuolated 

 and resemble those of the Eicheldarm of Balanoglossus ; in the terminal portion 

 is a "cartilaginous" or " gelatinoid " substance (" Stiitzkorper " of Schepotieff, 1905, 

 p. 800) which stains very darkly with haematoxylin. The notochord lies in the 

 region where the median septum of the dorsal part of the collar meets the septum 

 that divides the collar cavities from the proboscis cavity. 



The central nerve mass of Rhabdopleura (Fowler and Schepotieff) lies, as in 

 Cephalodiscus, on the dorsal side of the neck region. It is situated between the 

 plume-bases, the collar pores, and the anal papilla. The superficial epidermal cells 

 that cover it are pigmented, and, according to Schepotieff, constitute a sense organ. 



On the evidence of Fowler and Schepotieff the divisions of the coelom are 

 exactly as in Cephalodiscus, namely, a proboscis cavity in the buccal shield, a pair 

 of collar cavities and a pair of cavities in the trunk region. The pericardium, on 

 the authority of Schepotieff, constitutes a sixth cavity (vide postea). The cavity 

 in the buccal shield is shut' off from the collar coelom by a septum, and from this 

 septum there pass muscle fibres to the thickened ventral wall of the shield. The 

 cavity opens to the exterior by a pair of pores, the proboscis pores (SchepotiefF, 1905, 

 pp. 797 and 798, and fig. 2, Ksp.}. Situated in the proboscis cavity, and set close 

 against the septum, is the pericardium, within which is the heart, in contact with the 

 ventral surface of the notochord (SchepotiefF, p. 799, and fig. 4 ; cf. text-fig. 12 of 

 this paper). 



The right and left collar cavities are separated by a median septum dorsally to 

 the notochord, and are continued laterally into the flaps of the postoral lamella, and 

 forwards into the plumes and pinnules. In his earlier paper (1904, p. 16) Schepotieff 

 states that the plume cavity is separate from the collar cavity, but in the later one 

 (1905, p. 799) he regards the two as continuous. Each collar cavity opens to the 

 exterior by a collar canal, with a wide internal opening and a narrow collar pore. 

 While the collar pores of Cephalodiscus are strictly lateral, and far back, near the 

 gill-slit, they occur dorsally or dorso-laterally in Rhabdopleura, on the plume-bases, 

 on the right and left sides of the central nerve mass. Schepotieff represents the 

 collar pores close to the anus, whereas Fowler identifies the positions of the collar 

 pores with those of the ciliated tubercles of Sars, which in the earlier figures (Sars 

 and Lankester) are represented as at some distance in advance of the anus. The 

 difference is probably to be accounted for by the relative state of contraction or 

 extension of the polypide : in a well-extended polypide the lengthening of the neck 



