ANATOMY AND DEVELOPMENT OP PERIPAT[JS OAPENSIS. 235 



be described as tetrahedral. It is rather of a flattened oval 

 form, and cor.sists, as shown in sections (PI. XVII, fig. 19 c and 

 D, d), mainly of ganglion cells. In its interior is a cavity with 

 a distinct bounding membrane : the cells of which it is com- 

 posed vary somewhat in size, being smallest near the point of 

 attachment. At its free end is placed a highly refractive, 

 somewhat oval body, probably forming what Grube describes 

 as a dark spot, half embedded in its substance, and kept in 

 place by the sheath of nervous matter surrounding it. This 

 body appears to have fallen out in my sections. The whole 

 structure is attached to the under surface of the brain by a 

 very short stalk formed of a bundle of cells and nervous fibres. 



It is difficult to offer any interpretation of the nature of this 

 body. It is removed considerably from the surfi^ce of the animal, 

 and is not, therefore, so far as I can see, adapted to serve as an 

 organ of hearing. 



The distribution of the white or fibrous matter of the ganglia 

 is not very easy to describe. 



There is a central lobe of white matter (fig. 19 e), which 

 is continuous from ganglion to ganglion, where the two are 

 united. It is smaller behind than in front. On its ventral side 

 it exhibits fairly well-marked transverse commissural fibres, 

 connecting the two halves of the ganglion. Laterally and 

 somewhat ventrally it is prolonged into a horn (fig. 19 d, e, 5), 

 which I propose calling the ventro-lateral horn. In front it is 

 placed in a distinct protuberance of the brain, which is placed 

 ventrally to and nearly in the same vertical plane as the optic 

 nerve. This protuberance is best shown in the view of the 

 brain from below given in PI. XVIII, fig. 22. This part of the 

 horn is characterised by the presence of large vertically-directed 

 bundles of nerve-fibres, shown in transverse section in fig. 19 d. 

 Posteriorly the diameter of this horn is larger than in front 

 (fig. 19 E, F, g), but does not give rise to a protuberance on 

 the surface of the brain owing to the smaller development of 

 the median lobe behind. 



The median lobe of the brain is also prolonged into a dorso- 

 lateral lobe (fig. 19, a), which, as already mentioned, is freely 



