78 



The osphradium of Buccinum is usually obvious as 

 soon as the shell is removed, for it can be seen through 

 the mantle. There is no difficulty in finding it when the 

 mantle cavity is opened and the roof examined. It 

 stands out as a deep brown-black organ, situated right 

 across the gutter of the pallia! siphon and between this 

 organ and the gill (fig. 8, Osph.). It lies, therefore, on 

 the opposite side of the ctenidial axis, and by reason of 

 its build was formerly known as the " false gill." It is 

 long, and is made up of about 90-100 leaflets arranged 

 on both sides of an axis, which is nothing but an 

 elongated nerve ganglion covered by epithelium. These 

 leaflets are largest in the centre and diminish in size 

 towards the ends, giving the whole the appearance of an 

 elongated oval. The leaflets are roughly triangular in 

 shape, those of the right or ctenidial side being, however, 

 larger than those of the left (fig. 44). Each leaflet has 

 an inferior free edge (fig. 44, Inf.), a lateral edge (fig. 44, 

 Lat.), and what must be termed the third side of the 

 triangle — the internal edge — which is curved and fused 

 to both mantle and nerve axis (fig. 44, Pall and N. ax.). 



The entire surface of the lateral faces is pigmented, 

 but the pigment is often free from the inferior edges of 

 the leaflets, a condition the reverse of that described by 

 Bernard for Cassidaria. For a further study of the organ 

 reference must be made to sections. 



Each Osphradial leaflet is a fold of the epithelial 

 layer bounding the inner surface of the mantle, 

 together with certain connective supporting tissues. 

 Just as in the branchial leaflets, there is an external 

 bounding epithelium resting upon supporting mem- 

 branes and leaving a series of flat blood spaces, 

 which are here occupied largely by branches of the 

 osphradial nerves (fig. 45). The epithelium resembles 



