HORN ELL ANATOMY OF PLACUNA 81 



abnormal, arrangement of vessels noted in an individual obtained in 1907 from a 

 muddy inlet near the town of Tuticorin in South India. Here we see the rectal sinus 

 opening directly into a vessel, which, without doubt, is morphologically the right 

 auricle, but in this case with no connection whatever with the ventricle, inter- 

 communicating instead by a wide channel, the inter-auricular passage, with the left 

 auricle, and connected anteriorly with the right common efferent branchial trunk, 

 wherefrom it receives blood in the usual manner of a right auricle. In this individual 

 the ventricle has but two openings, that from the left auricle and that from the aorta 



EXCRETORY SYSTEM. 



The excretory system is considerably removed from the normal Lamellibranch 

 type, a result of the absence of a pericardium and of the marked asymmetry of the 

 animal generally. 



It consists of two paired asymmetric nephridia, connected dorsally by an extremely 

 short and wide transverse channel, the inter-nephridial passage. Two distinct regions 

 are recognisable in each nephriclium, an anterior section consisting of a laterally 

 flattened tubular region, with much folded walls lying anterior to the inter-nephridial 

 passage, and approximately of equal size in both nephridia, and of a caecal continuation 

 directed posteriorly, of great length in the case of the right nephridium, short, and 

 little more than rudimentary in that of the left (figs. 6, 7, and 8). 



The general form and course may be readily understood if we represent the organ 

 diagrammatically as an \\ -shaped tubular organ with the posterior part of the left leg 

 much shorter than its fellow on the right, thus l-j. It lies curved in crescent form 

 round the greater part of the adductor muscle, the anterior symmetric horns or 

 branches curving downwards over the anterior face of this muscle, while the posterior 

 horn on the right, representing the csecal prolongation of the light nephridium, lies 

 upon the upper surface and along the greater part of the rectum. The caecum of the 

 left nephridium appears as the abbreviated horn on the left. 



The two anterior branches of the conjoined nephridia (Neph.r. and Neph.l.), 

 usually of a dark brown colour, as are also the other sections, run parallel with each 

 other along each edge of the an tero- dorsal curvature of the adductor muscle, and 

 thence downwards along its anterior face, terminating about the level of the parieto- 

 splanchnic ganglionic mass (fig. 4). Dorsally the two tubes communicate widely with 

 their posterior caecal prolongations. 



The right branch in the anterior or symmetric region of the kidney is rather 

 broader than the left, and is more deeply lobed ventrally ; viewed superficially it 

 has the appearance of being partly sunk in the tissues of the right mantle. It is not so 

 in reality. Its posterior face is intimately associated with the proximal portion of the 

 pyloric caecum to which it is bound by a thin sheet of connective tissue. From its 



