22 



successively smaller as either end of the ctenidium is 

 approached. 



These leaflets are roughly triangular in shape. The 

 axis of the ctenidium lies close to the osphradium (fig. 8, 

 Ct. ax.). The respiratory leaflets are disposed along one 

 side only and are further attached by the whole of their 

 bases to the mantle (see Text-fig. 3, p. 25). 



The ctenidium is therefore Monopectinate. 



The efferent branchial vessel runs along under the 

 ctenidial axis and thus the area of each leaflet adjoining 

 the axis may be known as the efferent end and the free 

 side, the efferent margin of each leaflet. Branchial 

 lacunae extend up the afferent margins of the leaflets from 

 the afferent branchial sinus which lies in the mantle 

 immediately below the point of attachment of the afferent 

 edges of the lamellae. 



The Ctenidial axis (fig. 8, Ct. ax.) is conspicuous as a 

 smooth ridge running along the inner surface of the 

 mantle as far as there are ctenidial leaflets. 



It is made of the same peculiarly vacuolated con- 

 nective tissue (fig. 39, Ct. gl.) seen in the mantle at the 

 base of the ctenidial leaflets. Towards the side to which 

 the branchial lamellae are attached the place of this 

 tissue is taken by longitudinal muscle fibres (fig. 39, 

 Ct. mus.). This layer increases in thickness towards the 

 middle of the ridge. 



The ctenidial nerve (fig. 39, Ct. n.) runs along the 

 axis not far from the osphradial side, and gives off 

 branches at intervals, which pass to the leaflets. 



Histology. 



The microscopic structure of the ctenidial leaflets 

 is interesting by reason of the histological differentiation 

 of the different areas. Each leaflet consists of a double 



