VIII. COLLAR (including the arms and the opercukim). 



This division of the animal is not only one of the most characteristic and important 

 parts of Cep halodiscus ^ but it is also the most difficuk to understand thoroughly. The relations 

 of its several parts, and particiilarly of the arms to the opercukim, have not previously been 

 adequately described. 



It will be remembered that in Balanoglossus the anterior margin of the collar forms a 

 projecting fold encircling the base of the proboscis-stalk. The ventral half of this fold may be 

 regarded as constituting a lower lip, while the dorsal part is connected, in the middle line, with 

 the anterior neuropore. In Cephalodisctcs the neuropore is not represented, and the collar forms 

 no projection in the median dorsal line above the base of the proboscis. Except for this interval, 

 the whole of the anterior margin of the collar forms a strongly-developed fold, split up dorsally 

 to form the arms, and ventrally constituting the operculum. It is necessary to insist on this last 

 point because the base of the operculum is so narrow, and the flap itself is so commonly 

 directed away from the proboscis, that the operculum might be taken to be a derivative of the 

 posterior edge of the collar, as indeed I at one time (87, p. 43) supposed it to be. lts real 

 relations can be most easily seen in C. leviiiseni (PI. IV, figs. 34 — 36), in which the median 

 ventral part of the collar-cavity is so long that it is at once apparent that the operculum is 

 developed from its anterior margin. Unless this be recognized, it is impossible to form a clear 

 conception of the relations of the arms to the operculum. 



Fig. 25 (PI. III) represents part of a plasticine reconstruction of C. gracilis, made from 

 tke series of obliquely sagittal sections some of wkich are shewn in PI. V, figs. 43 — 53. 



The ventral lobe of the proboscis of the reconstruction has, however, been turned 

 dorsally, and the operculum itself has been folded in the opposite direction, in order to e.xpose 

 the mouth (/;/ ). The bases of the five arms characteristic of this species are shewn, the first 

 and second arms i^R. i, R. 2) having become free from the anterior collar-fold, while the third, 

 fourth and hfth arms {R. 3 — 5) are not yet free, but are indicated \)\ their food-grooves turned 

 towards the mouth, and by their angular dorsal or external surfaces. The right lateral lobe 

 of the operculum (c/i.) is kere shewn in section, exposing the corresponding part of the 

 collar-cavity. 



The arm-bases form a gentle curve which is convex towards the mouth. The effect of 

 this arrangement is that while the food-groove of the first arm faces al most dorsally, those of 

 the second, third and fourth arms face anteriorly, and that of the fifth arm nearly ventrally. 

 In passing away from the middle line, the fifth arm (in this specimen) rapidly becomes twisted 

 round its own longitudinal axis, so that in the reconstruction fig. 22 (taken from the same 

 specimen, but shewing more external portions of the arms and the whole of the right side of 

 the operculum) its food-groove faces directly backwards, or towards the anus. The whole 

 arrangement of the parts round the mouth shews that the proboscis and the anterior fold of 

 the collar together form a highly efficiënt apparatus for conducting food to the mouth. The 

 Diatoms and other minute structures which are actually found in the alimentary canal are 



