RHABDOPLEURA AND CEPHALODISCUS. 17 



The mode of development of the plumes of Rliahdopleura 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, pi. 39, fig. 3, buds 5 and 6, and the slightly 

 older bud figured below these ; also Allman, 1869, pi. 8, figs. 7 and 8, and Schepotieff, 

 1905, p. 802, fig. 6) renders it almost certain that these two plumes are the ecpiva- 

 lents of the first pair of plumes that develop upon the bud of Cephalodiseus, and not 

 of the short lophophoral arm which in Cephalodiseus dodecalophus bears the six 

 plumes of its own side of the body. The second and later pairs of plumes of Cephalo- 

 diseus are not represented in Rhabdoplmi^a. The pinnules of Rhabdopleura are 

 equivalent to the pinnules of Cephalodiseus. 



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) Bchepotieft' 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 (1905, p. 797) he has presumably withdrawn that view.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 Cephalodiseus 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 

 Cephalodiseus. 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 (" Kiemeurinnen " 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 Cephalodiseus which Masterman 

 has described as conducting the food particles from the plumes into the mouth. In 

 Cephalodiseus 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 Rhabdopleura. 

 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, i.e., at the anterior end of the pleurochord, that one would 

 look for the gill-slit. 



