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 pleurochords 
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 (‘“ Stiitzkérper” 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 
