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W. J. Dakin 
osphradium above the branchial nerve. Along this nerve are many ganglion 
cells, particularly on the osphradial side and amongst these occur bipolar 
cells (fig. 25 n.c). One process from these cells appears to enter the nerve, 
the other runs up to the epithelium of the sense organ and may branch 
before entering the epithelium or pass directly between the cells of this 
layer. In any case the neurofibrillae enter the osphradium, running almost 
perpendicularly to the surface of the epithelium and reach eventually the 
cuticular region. Near the cuticle they break up into very delicate fibrilla e 
which appear to become connected by transverse branches forming a 
subcuticular network. (It has only been with the greatest difficulty 
that this anastomosis was made out.) Finally from this net, fibrils pass 
through the cuticle but most of these do not project above the surface 
at all. There is therefore no covering of sense hairs to the osphradium. 
The system of neurofibrillae in this sense organ resembles the nerve 
endings found in the skin of Lumbricus by Dechant (1906). 
Summary. 
1. The visceral ganglia of Pecten are fused forming a single largo 
ganglion which is situated in the usuai position on the ventral surface 
of the adductor muscle. It is complicated in form and structure and is 
probably unique in the lamellibranchiata. 
2. The great development of the visceral ganglion of Pecten is to be 
correlated with the extent of the area of Innervation ,and due to the 
reduction of the more anterior parts of the animai, the absence altogether 
of the anterior adductor, and the corresponding increase in size of the 
posterior adductor muscle and the posterior parts of the animai generally. 
3. The ganglion is divided into lobes, certain of which are pigmented. 
On the ventral surface, a large hemispherical ventro-central lobe is vi- 
sible, connected by two depressed areas, the lateral grooves, with two 
lateral lobes. The lateral lobes are unequal in size, that of the left side 
being larger than the right. Two pigmented lobes which occupy a central 
Position on the dorsal side of the ganglion, the dorso-central lobes, are 
also visible from the ventral surface. The nerves arise from the lateral 
lobes or the lateral grooves and all with the exception of two osphradio- 
branchial nerves arise from the former, chiefly from the dorsal surface. 
4. The osphradium in Pecten is innervated by fibres from three 
nerves on each side, the branchial, the osphradio-branchial and a poste- 
rior nerve. All arise from the visceral ganglion but the two former contain 
