40 



bounded by the usual epidermis formed of a single layer 

 of epithelial cells, amongst which sense cells occur 

 frequently on the sides. 



Under the epithelium the connective tissue is more 

 compact and dense than elsewhere with the series of muscle 

 fibres (fig. 45, Br. m! , Br. m.") which have been already 

 described. Vascular spaces are of frequent occurrence in 

 the connective tissue. 



Near the margin to which the gill filaments are 

 attached, and running in the same direction as the 

 ctenidial axis, are the branchial nerve (iig. 45, N . Br.) the 

 afferent branchial vessel and the efferent branchial vessel 

 occurring in the order named (iig. 45, Br. aff ., Br. eff' .), 

 the nerve being nearest to the body and the efferent 

 branchial vessel to the gill filaments. The two blood 

 vessels are very close together, and ai^e only separated by 

 a narrow bridge of connective tissue. From the two 

 corners of the afferent branchial vessel nearest to the gill 

 filaments (it is almost rectangular in section) branches are 

 given off which pass betw'een the wall of the efferent vessel 

 and the surface, and open into expansions on certain of 

 the filaments. 



The filaments which make up the lamellae are hollow 

 outgrowths from the axis, and arise as simple, straight 

 processes, becoming reflected later. Jackson found in 

 P. irr-adians that the young forms examined had 

 comparatively a much shorter reflected portion than the 

 adults. 



The gills of Pecten are amongst the best examples 

 known of the j^l^cate type, that is, the filaments, instead 

 of being arranged in a flat uniform series, are so placed 

 that the lamellae are thrown into a series of vertical folds 

 or plicae (fig. 22, and Text-fig. 2). This plication is most 

 obvious near the ctenidial axis where the folds are so 



