HORNELL- AN ATOMY OF PLACUNA 69 



the renal organ readily distinguishable even to the naked eye by reason of the dark 

 colour of its glandular tissue. 



The intestine is lined throughout its entire length with ciliated epithelium, and its 

 absorbing area is considerably increased by the presence of a typhlosole or longitudinal 

 ridge extending to the anus from about the first fifth of the intestine (fig. 26, Ty.). 



Anteriorly the typhlosole when seen in section is somewhat mushroom-shaped, a 

 narrow neck bearing a button-shaped " head." It is much less prominent and less 

 expanded than in Margariiifera vulgaris in the corresponding region. Posteriorly 

 it is even simpler, consisting of a simple parallel sided fold projecting into the 

 interior of the rectum to about its semi-diameter. 



In common with Anomia and Area, the rectum is in no way attached or connected 

 with the ventricle of the heart. It passes quite free on the anterior and ventral 

 aspects of the heart. 



BRANCHIAL SYSTEM. 



The branchiae are long and greatly curved, four delicately graved scimitars lying 

 within the protection of the free portions of the mantle (fig. 4). From between the 

 ventral edges of the labial palps the branchiae curve first downwards and then 

 backwards to a point just ventral to the anal aperture, where all four close together 

 somewhat abruptly to a sharply attenuated conjoint free tip. In their course they 

 keep parallel and a little inward to the mantle edge, the inner curve half circling the 

 adductor muscle being suspended by two deep curtains of thin tissue, the branchial 

 mesenteries, which hang from the renal tubes in the first part of their course, and in 

 the hinder region from the mantle close to its line of junction with the anterior and 

 ventral edges of the adductor muscle. 



The four branchiae or gills are disposed in pairs on either side of the body. As 

 each pair constitutes a ctenidium, each branchia or gill is morphologically a hemi- 

 ctenidium. 



The ctenidia show comparative simplicity of structure. There is the usual 

 longitudinal vascular base or axis along the inner margin of each ctenidium, upon 

 which, on the outer face, are inserted two parallel series of long and delicate tubular 

 outgrowths, the branchial filaments. 



All the filaments being inserted at right angles to the axis on the outer face, 

 their initial direction is outwards and towards the nearest section of the pallial margin. 

 At a common distance from the vascular axis, filaments in each series change their 

 course those of the outer, or external, series are reflected or folded outwards upon 

 themselves in the form of a V, with closely approximated wings, while those of the 

 internal series are similarly V-folded, but in their case the folding is inwards 

 (figs. 12 and 24). 



Each series of filaments constituting a branchia is now seen to be divisible into 



G 2 



