MORPHOLOGY OF MELIBE 561 



precisely reverses this action. Rarely excretory cells do both, 

 but even then an excretory cell absorbs the one substance 

 more freely than the other, or vice versa. These two types 

 are associated with voluminous organs. The indigo kidneys 

 produce urea, uric acid, and urates, while in carminate kidneys, 

 thus far known, none of these substances are formed, though 

 some non-indigo excretory cells contain urates. Referring to 

 special cases he states : ' In two groups of molluscs the nephridia 

 instead of being lined throughout their entire extent by a 

 single type of excretory cells present noteworthy differences : 

 in Amphineura the reno-pericardial ducts of acid reaction 

 eliminate actively carminate and litmus ; while the rest of the 

 nephridium, formed of different cells, and with alkaline reaction, 

 eliminate indigo. Both nephridia of Patella eliminate 

 equally indigo. The most numerous non-ciliate eliminate 

 indigo ; the others, ciliated, eliminate only carminate — the 

 single nephridium being thus a physiological equivalent of two 

 nephridia in the Diotocardia (Trochus, &c.).' 



9. The Organs of Reproduction. 



It was pointed out by Lankester (1881, 1893), Sedgwick 

 (1888, 1898), Goodrich (1895), and Lang (1896), that the true 

 coelom in molluscs is much reduced, being divided into three 

 parts : (1) the pericardium, (2) the perigonadium, and (3) the 

 nephrocoel ; the remaining body-cavities being haemocoels 

 which are derived in part from a system of spaces which arise 

 between the ectoderm and the entoderm (Sedgwick, 1888 : 

 383). It is not my purpose to discuss here the homology of these 

 cavities in M. leonina, but only to call attention to their 

 relation to one another and their relative duct systems. As 

 stated above the pericardium is a closed cavity which com- 

 municates with the nephrocoel by the renal syrinx which seems 

 to be closed at its point of communication by what I have 

 called a cyncitial plate. The perigonadial coelom lies directly 

 below the nephrocoel. It was shown by Erlanger, 1891-2, 

 for Paludina that the pericardium arises as two coelomic 

 sacs on either side by a hollowing out of the mesoblast. 



NO. 268 P p 



