00 OKHAMANDAL MARINE ZOOLOGY REPORT 



harmful influence of the mud whereon the shell rests. The reason for the animal 

 lying on its convex side is unmistakeably the same. As mud-laden water enters the 

 shell to pass to the gills, the heavier particles tend to settle and deposit, and it becomes 

 a manifest advantage to have the organs on the aspect above where this settling takes 

 place. The scallops (Pectcn maximus especially) and the edible oyster (Ostrea) supply 

 analogies of the advantage to unattached animals, whose life axis is horizontal, to have 

 one of their valves convex upon which they may lie. Pectcn rests thus, while in the 

 case of the oyster (Ostrea), which in the natural condition, living attached to rocks, is 

 attached by its left or convex valve, those that have been detached for cultural 

 purposes are preferably laid down upon this convex valve, as this is found distinctly 

 beneficial. 



In all Lamollibranchs, with the exception of the Anomiidcse, the excretory system 

 opens on the one hand to the exterior and on the other into a definite portion of the 

 ccelom the pericardium. In the Anomiideaj, typified by the genera Anomia and 

 Placuna, the absence of a pericardium is characteristic, and renders them unique 

 amongst Lamcllibranchs. 



It was only in 1903 that Sassi (loc. cit.) was able to show that minute vestiges 

 of the crelomic cavity do persist in Anomia in the form of tiny blind tubules 

 communicating with each kidney, but without relationship to the heart. Sassi at the 

 same time showed that the transverse tube, or interncphridial passage connecting the 

 kidneys, is not of ccelomic value, being lined with cells similar to the secretory cells 

 in other parts of the renal organ. 



In Placuna there is, as we have seen, a wide duct connecting the right with the 

 left kidney. This is, undoubtedly, strictly homologous with the transverse renal 

 passage found by Sassi in Anomia ; in both the passage lies anterior to, and at the 

 level of, the ventricle, and between the crystalline style sac and the rectum ; in both, 

 the cell structure of the wall is similar to that of the general renal surface. This last 

 fact compels us to look elsewhere in Placuna for the vestige of a coelomic cavity. 



I have been able to find nothing similar to the two groups of blind ciliated funnels 

 (" Wimpertrichtern ") described by Sassi as representing minute vestiges of the coelom, 

 but I am strongly inclined to the belief that the pair of small triangular glandular 

 organs, one of which is situated superficially on each side at the anterior ventral corner 

 of the visceral mass, and which open into the most dorsal section of each kidney, are 

 comparable to them and probably homologous. As in Sassi's funnels, in Anomia they 

 lie in front of the passage connecting the kidneys ; likewise they open directly into 

 these organs. 



In Placuna these glandular bodies are much larger, more compact, and more 

 conspicuous than those in Anomia. They appear functionally to be of superior 

 importance, and exhibit a suggestive resemblance to the dark-lined pericardial glands 

 (Keber's organ) so frequently met with in other Lame Hi branch s, as, for example, the 



