PTEROBRANCHIA OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 427 



seen on the right side. This section shows the large right lobe of the operculum in a 

 region where it is free from the rest of the collar. The species appears to be charac- 

 terised by having these free lateral lobes of the operculum well developed. It may 

 be supposed that the size of the lateral lobes is to some extent correlated with the 

 number of the arms ; since it can hardly be doubted that one of the most important 

 functions of the operculum is to help to convert the ventral grooves of the arms into 

 channels leading the food to the mouth. A large development of the arms would 

 accordingly require a corresponding development of the opercular lobes, which, during 

 the act of feeding, would presumably be thrown into the position seen on the right 

 side in fig. 8. 



Comparison with other species of Cephalodiscus in this respect cannot be 

 attempted very satisfactorily, but it may be noted that the operculum of C. hodgsoni, 

 figured by RIDEWOOD (07 l , pi. vi. fig. 53), has a close resemblance to that of 

 C. agglutinans, here described. Except with the aid of solid reconstructions, which 

 have not been made in many cases, it is often difficult to ascertain the exact form of 

 this organ ; but it may be remarked that in C. gracilis, of which a plasticine reconstruc- 

 tion has been figured in pi. iii. fig. 25, of the Siboga report (HARMER, 05), the 

 operculum is hardly emarginate in the middle line, and has no special development 

 of its lateral lobes. The arms in that species are comparatively few, only five being 

 present on each side. 



Two further points may be noted with regard to the operculum of C. agglutinans : 



(a) The last arm of each side is continuous with the corresponding opercular 

 lobe, as in other species of Cephalodiscus (cf. HARMER, 05, pp. 31, 36 ; and RIDEWOOD, 

 07 2 . p. 229). This is sufficiently illustrated by fig. 10. 



(b) The free edge of the operculum is more or less scalloped, as is indicated by 

 fig. 11, where two of the end-lobes are cut separately. Evidence that the edge has 

 this form has also been obtained in some of the dissected specimens. The same 

 character has been figured by RIDEWOOD (07 1 , pi. vi. fig. 53) in C. hodgsoni. 



(iii.) Collar-canals. 



These structures agree in general form and position with those of other species of 

 the genus. Seen in side view the ventral outline is very convex, while the dorsal 

 outline may be concave. In a frontal section of the zooid (fig. 11), a collar-canal 

 which is cut near its dorsal side may appear as two separate parts. The inner part 

 (c.c.i.) opens into the collar-cavity, while the two parts of the tube are connected by 

 the strong collar-muscle. The epithelium of the canal is much thinner on the dorsal 

 side than it is ventrally ; and it is reasonable to suppose that it is more fiexible there. 

 The muscle would thus seem to have the function partly of dilating the coelomic 

 opening and partly of acting in antagonism to the prolongation of the principal 

 muscular mass of the metasome ; which, as shown in the sagittal section, fig. 16, ms., 



(ROY. soc. EDIN. TRANS., VOL. XLIX., 551.) 



