360 DR W. CARMICHAEL M'lNTOSH ON THE STRUCTURE OF THE 



ments of the " brain" in Tetrastemma and Borlasia, they do not explain the dis- 

 tinction in regard to the position of the nerve-trunks in these species. Prof. Kefer- 

 stein likewise did not observe this essential distinction between the two groups, 

 but considered Cerebratulus the type of the whole. He describes an otolite or 

 two in the middle of the ganglion in a young (Erstedia pallida, but I fear such 

 are only pigment-granules and cells, or collections of oil. E. Gkaeffe,* again, in 

 some brief remarks on a Tetrastemma from Nice, states that he found a small 

 cluster of otolite-capsules between the eyes, each capsule containing a crowd of 

 minute otolites. If such were not pigment-cells or structures pertaining to the 

 cephalic sacs, the Mediterranean form shows a most interesting advance on the 

 British in this respect, as well as in having lenses to its eyes. Unfortunately, 

 the author has not figured the structures. 



The only British Ommatoplean, so far as I have seen, which shows a special 

 structure in its eye-specks, is 0. pulchra. In this species the pigment is grouped 

 within a distinct capsule (Plate VII. fig. 8, from a dead, and therefore slightly 

 injured specimen). The eyes in the living animal have a clear patch in the centre, 

 from the projection of the lens-like capsule. In 0. gracilis and others, a few of the 

 eye- specks are frequently connected together by bridges of the pigmentary sub- 

 stance. Though a pale portion is sometimes seen in the specks of the former, I 

 have not satisfactorily made out a lenticular structure. In Tetrastemma vermi- 

 culus, which has frequently been sent me from St Andrews, the eyes of each side 

 are connected by a longitudinal patch of dark pigment, so that in contraction 

 the animal seems only to have two large crescentic eyes, of a very characteristic 

 appearance. 



Cephalic Sacs and Furrows. — Midway between the tip of the snout and the 

 anterior border of the ganglion in O. alba, a furrow runs inwards and slightly 

 forwards on the dorsum, ceasing, however, before the middle line is reached; and 

 on the ventral surface a similar though shorter furrow exists, the two meeting in 

 a dimple, furnished with longer cilia, on the side (where the cilia are more active and 

 powerful than usual), which depression leads into the cephalic sac. A short distance 

 behind the ganglia two other superficial furrows occur, each slanting backwards and 

 inwards to meet its fellow of the opposite side in the middle line. These furrows 

 are also continued inferiorly, but with a slightly different direction, so that they 

 meet under the ganglia. The two sets of furrows are very distinctly marked in a 

 flattened head as lateral notches. From the dimple mentioned in connection with 

 the anterior furrows, a thick- walled ciliated duct on each side leads into a con- 

 siderable ovoid, pyriform, or almond-shaped glandular mass, which lies in front 

 of and rather exterior to the ganglion of the side (Plate VI. fig. 1, m)\ and from 

 what is seen in translucent species, such as Tetrastemma vermiculus, it would 

 appear to end in a cul-de-sac , the walls, moreover, under pressure are marked 



* Beobach. iiber Rad. und Wiirmer in Nizza, Zurich, 1858. 



