In fio-. 47, the right arms (thosr shewn in the reconstructions fius. 22, 25) are becoming 

 appaivnt. W'hile the precedin^^- section (fig. 46) has 110 tracé of food-grooves, the present 

 section shews the conimencement of the groeves (/\'. i, /v. 2} of the first two arms of the 

 right side. 



The structure of the remaining sections (figs. 48 — 53) vvill be clear with the aid of the 

 reconstructions, hgs. 22 and 25. It will be noticed that the food-grooves 2—5 successively 

 make their appearance-, and it is clear that as these sections are transverse to the grooves, 

 the fact that the latter are not traceable to the mouth indicates that they die away in 

 approaching that aperture. It will be seen that each of the arms passes directly outwards for 

 some little distance from its base, as is implied by the fact that near the middle line of the 

 animal the arms are cut transversely. This position corresponds closely with that in which the 

 arms are developed in the bud (cf. hg. 32); and although the arms are of coiirse able to 

 place themselves in a great variet)- of attitudes, they ha]jpen to be in what may be regarded 

 as their normal position on the right side of this individual. After passing outwards for some 

 distance, they for the most part turn dorsally, as shewn for J\. 1 in figs. 52, 53, and for J\. i 

 and 7?. 2 in fig. 22. They may at the same time be twisted about their own longitudinal axis, 

 as indicated for 7?. 5 in figs. 25 and 22. 



Tracino- the arms from the outer side tovvards the mouth it will be seen that the food- 

 CTrooves fade away as they pass into a tunnel-like space bounded by the anterior wall of the 

 arm-base, the operculum and the posterior wall of the proboscis. This tunnel curves round the 

 proboscis-stalk until it leads to the mouth. 



The figures further demonstrate the existence of the lateral lobe of the operculum, 

 which projects a considerable distance externally to the point where the arms become free. 

 Fig. 5 I shews the level at which this lateral lobe begins to be free. It is indeed still connected 

 with the base of the fifth arm, but its cavity is disconnected from that of the arm, since the 

 section merely cuts the marginal epidermis of the part of the operculuni which immediately 

 adjoins the arm. In the more external sections (figs. 52 and 53) the operculum is quite separate 

 from the arms. One further point may be noticed in this series of sections. The coUar-canal 

 {i\ c.) is seen to be a short tube which opens into the collar-cavity at the end situated nearer 

 the middle line of the collar, and to the exlerior at its more external end. By the time the 

 external opening is reached, the collar-cavity itself has disappeared ; or, in other words, the 

 collar-canal occupies a lateral recess of the collar-cavity. 



The number of arms, in this as in other species of Cephalodiscus^ appears to be in the 

 main constant for each species. It is sometimes, however, no easy task to count the arms in 

 sections which are not favourably orientated, while results based on entire preparations are 

 uncertain. In one series of sections of C. gracilis, I have convinced myself ot the existence 

 of six arms on one side, instead of the normal five ; while in one case of C. dodecalopJius, I 

 have only been able to count five arms on one side. It would almost be expected that some 

 variation would occur in these respects. The variation might well make its appearance in the 

 bud bv the ereater or smaller amount of subdivision of the free dorsal edge of the collar. 



SIBOGA-SXPEDITIE XXVI (^/V. 5 



