RHABDOPLEURA AND CEPHALODISOUS. 17 
The mode of development of the plumes of Rhabdopleura from the dorsal region 
of the collar near the stalk of the buccal shield, and the mode of development of 
pinnules upon them (see Lankester, 13, pl. 39, fig. 8, buds 5 and 6, and the slightly 
older bud figured below these ; also Allman, 1869, pl. 8, figs. 7 and 8, and Schepotieff, 
1905, p. 802, fig. 6) renders it almost certain that these two plumes are the equiva- 
lents of the first pair of plumes that develop upon the bud of Cephalodiscus, and not 
of the short lophophoral arm which in Cephalodiscus dodecalophus bears the six 
plumes of its own side of the body. The second and later pairs of plumes of Cephalo- 
discus are not represented in Rhabdopleura. The pinnules of Rhabdopleura are 
equivalent to the pinnules of Cephalodiscus. 
On each plume of Rhabdopleura are about fifty pinnules, projecting ventrally 
from the two edges of a ciliated groove, which has a central ridge. In his earlier 
paper (1904, p. 12) Schepotieff was inclined to regard this central ridge as bearing 
a third row of pinnules, but since he says nothing of these in his later paper 
(1905, p. 797) he has presumably withdrawn that view. The plumes and their 
pinnules are hollow, and are lined by a tough sheet of skeletal connective tissue, 
which remains when the epithelium disintegrates, and gives a false impression of 
being solid; hence the erroneous statement of Sars that the skeleton of these parts 
is cartilaginous. 
The postoral lamella resembles that of Cephalodiscus in that it consists of a pair 
of lateral flaps (“‘ Seitenlippen ” of Schepotieff), containing continuations of the collar 
coelom, the middle part, posterior to the mouth, being, however, less free than in 
Cephalodiscus. Rhabdopleura has a front lip, distinct from the postoral lamella. 
Although gill-slits do not exist in Rhabdopleura, the position which these would 
occupy is clearly indicated by a pair of ciliated grooves (‘‘ Kiemenrinnen” of 
Schepotieff, 1904, pp. 13 and 14, and 1905, p. 796 and fig. 5). These grooves are 
clearly the equivalents of the pair of triple grooves of Cephalodiscus which Masterman 
has described as conducting the food particles from the plumes into the mouth. In 
Cephalodiscus there are a pair of perforations of the pharyngeal wall which serve 
the purpose of disposing of the excess of water that has come down the grooves 
charged with the food organisms. Such perforations are wanting in Ahabdopleura. 
Schepotieff’s Kiemenrinne is a continuation of the ciliated groove on the ventral 
side of the plume-axis; it passes from the plume-base down the side of the collar 
region and the stalk of the shield, and runs down the side of the mouth into the 
first portion of the alimentary canal, where its walls change in character. At the 
side of the mouth the cells composing the wall of the groove are tall, columnar, 
ciliated cells; those in the pharyngeal region are pale-staining cells, partially 
vacuolated, and sharply delimited from the other cells of the pharyngeal wall—they 
constitute the “ pleurochords” (Schepotieff, 1905, fig. 5). It is where the groove 
changes in character, 7.¢., at the anterior end of the pleurochord, that one would 
look for the gill-slit. 
