56 OKHA.MANDAI, MAKINK /OOLOGY REPORT 



Projecting forwards from cadi side of the anterior /ispeet nf the vise-mil mass i.s a 

 pair of lamellar palps (7V) as in other Lamellibranohs. In Placuna they arc greatly 

 elongated dorso-ventrally and narrow relatively to their length. The opposing faces of 

 the members of each pair arc deeply grooved transversely, and clothed with ciliated 

 epithelium. In common with the majority of organs in this species, the palps are 

 asymmetric, due to the mouth being placed to the right side of the median line. As a 

 consequence the upper and lower lips, which normally are prominent transverse folds 

 connecting the dorsal and ventral (outer and inner) palps of either side respectively, 

 are here twisted to the right, and come to assume rather a vertical than a transverse 

 position. The ventral margin of each inner palp bends inwards below the insertion of 

 the foot, and unites in the median line with its fellow from the opposite side. 



The anterior tip of each ctenidium or branchia is led into and attached to the base 

 of the gutter formed by approximation posteriorly of the bases of the two palps of its 

 respective side. Beginning from this attachment each ctenidium is suspended through- 

 out its length from a wide mesentery, attached along its inner margin for a short 

 extent anteriorly to the kidney, and for the remainder and -greater extent to the 

 mantle close to its junction with the adductor muscle in the case of the left mesentery, 

 and along the course of the stylar (ventral) visceral lobe in the case of the right 

 ctenidium. 



Cutting away the gills and their suspensory membrane or mesentery close to the 

 adductor muscle, together with the left pair of palps, we are enabled to see clearly how 

 greatly interpenetrated is the substance of the right mantle by various lobes of the 

 visceral organs. Beside the two hinge-lobes already mentioned, the principal of these 

 arc three in number, which may be termed respectively the posterior genital, the rectal, 

 and the stylar lobe (Plate III., fig. 13). 



The first of these is an elongated, somewhat wedge-shaped tumid mass (Go.p.}, 

 having its long axis directed obliquely posteriorly and ventrally ; a deep groove marks 

 it off from the posterior margin of the main visceral mass, connection being made by 

 a narrow bridge of tissue dorsal to the apex of the ventricle. 



The rectal lobe lies ventral to the last named ; it runs obliquely backwards from 

 the mid- ventral region of the main visceral mass at an angle of about fifty degrees from 

 the vertical, and terminates a little above and beyond. the posterior extremity of the 

 gills. For part of its course it is attached ventrally to the surface of the adductor 

 muscle, while near its distal extremity it is approximated to the ventral end of the 

 posterior genital lobe, to which it is connected by a very narrow and inconspicuous 

 bridge of tissue. The rectum lies for most of its course within this rectal visceral lobe ; 

 at its posterior extremity the anus opens in the centre of a wide and shallow funnel 

 (An.f., fig. 5). 



The third visceral lobe is peculiarly interesting, as although it has no great bulk 

 its length is relatively extreme. Its position may be defined as parallel and coincident 



