92 



and bears a somewhat crescentic patch of pigmented epi- 

 thelium (Ep.op.) ; the other Hp, which is turned away from 

 the wall of the siphon, is sharp, and is raised up to form a 

 screen. The nerve branch entering the tentacle from the 

 posterior pallial plexus runs in the axial part, and at the 

 summit swells out into a nearly globular ganglion {Ga.op.) 

 consisting of large, clear, oval cells and a network of fibres. 

 On the siphonal side of the tentacle this ganglion lies close 

 to the wall. It is invested on all sides except towards the 

 tip and the outer wall by a capsule of fibrous tissue, which 

 stains deeply and homogeneously with staining reagents, 

 has no nuclei, and shows little indication of cellular nature. 

 On the tip of the tentacle the hair cells, referred to above, 

 can be traced into the tissue of the ganglion. Nerve fibres 

 from the latter probably spread round the edge of the 

 incomplete fibrous capsule, not through its tissue. 



The cells covering the tentacle generally are cubical in 

 form with a continuous cuticle. Towards the tip, how- 

 ever, they become higher and columnar in shape, and the 

 cuticle becomes less evident. On the pigmented spot the 

 outer two-thirds of each of these columnar cells is filled up 

 with brownish pigment, which takes the form of a dense 

 mass of spherical granules lying perfectly free from each 

 other. The nucleus is in contact with the lower layers of 

 this mass of granules, but is usually quite free. The cell 

 body seem to be composed of clear cell substance, in which 

 are embedded the round pigment granules. Beneath the 

 nucleus it is faintly fibrillar, the direction of the fibrillae 

 being that of the long axis of the cell. A narrow space 

 separates this epithelium from the capsule surrounding 

 the ganglion, and this space is filled by fine fibrous tissue. 

 It is very probable that nerve fibres from the ganglion, 

 passing round the edge of the capsule, form part of this 

 layer, and some at least terminate in or among the cells 



