3^' 



collar-cavity of this region. The septum has, however, a free internal edge (cf PI. XI, lig. 140), 

 so that the arm-cavities become successively continuous with the general collar-cavity. 



The plane of the sections is not well suited to demonstrate the relation between the 

 arms and the operculum: Imi it is sufficiently obvious that the base of the operculum is cut 

 immediately after the disappearance of the sixth arm on either side (figs. 118, 119), and that 

 in fact it is the last arm on each side which becomes continuous with the operculum. It will 

 be remembered that the operculum is an exceedingly mobile organ, which can place itself in 

 a great variety of positions. In this particular specimen the left side of the operculum is in the 

 main standing at right angles to the ])roboscis-stalk, but with some forward direction. On the 

 right side, the ^percuhun is directed backwards over tlie region of the collar-pore and gill-slit. 

 It thus results that in tracing the scries backwards, the oi)erculum appears first on the left .side 

 (figs. 116 — 118); it is later cut tangentially on the same side (figs. 119 — 121), although the 

 sections are complicated by the presence of several folds in the organ ; while on the right side 

 (figs. 118 — 123) the sections of the operculum have a simple character, owing to the fact that 

 it is hansfinor ventrallv and backwards from the moLith. 



The beginning of the right half of the o])erculum is indicated in fig. 118 by a ridge 

 of basement-membrane {b. w.) which jirojects into the right collar-cavity. This ridge resembles 

 those which form the commencements of the arm-cavities, and like them has a free internal 

 edge. The separation of the oj)erculum from the rest of the collar is seen, on the left side, in 

 fig. 119 by projections of the basement-membrane (d. iii.) developed from both the dorsal and 

 the ventral sides of the collar-cavity. The basement-membrane which lines the operculum is 

 quite thin, which is in marked contrast with the condition found in the main part of the collar- 

 cavity and in the arm-bases (figs. 119 — 122). It may be presumed that the basement-membrane 

 has a supporting value, and that its great development in the arm-bases is connected with the 

 fact that these parts are relatively stiff compared with ihc! mobile operculum. 



The later sections figured (figs. 120 — 124) shew the collar-cavities passing at the sides 

 of the niduth and finally uniting on the ventral side of that aperture (fig. 122). The collar- 

 canals (figs. 121, 122) are situated in the ventral end of that ])art of the collar-cavity which 

 is continued into the arm-base. 



The left series of arms shewn in figs, i 13 — i i 7 has an extremely characteristic arrangement, 

 which can also be made out on the right .side, although these arms are lying in a position 

 which is not quite so favourable for demonstrating their real relations. 



It will be seen from fig. 113 that while the arms A. i and L. 2 have their food-grooves 

 facing the proboscis, L. 3 faces directly outwards, and L. 4 — 6 face away from the proboscis. 

 A similar arrangement has been described by M.\sterm.\n (97, 2, p. 346, PI. XXVI, figs. 32 — 36; 

 98, 2, ]). 521, PI. IV, fig. 68) in C. dodccalopliiis, the diagrammatic figures shewing three of 

 the arms facing the proboscis and the other three witii tiieir backs to the proboscis. Now while 

 the account given in the earlier account is perfectly correct, in the second paper referred to 

 Masterman inverts the numbering, and moreover describes a process of rotation of the arms 

 which is .supposed to l)ring them into the position shewn in his figure. No mention is made of 

 the discrepancy between the two accounts, and I am unable to explain why Masterman 



