IX 



MOLLUSCA 313 



difficulty in the case of Paludina is that the complete series of 

 earlier stages is not at all easy to obtain, since the earlier stages of 

 development are passed through rapidly while the later stages of 

 growth take much longer to accomplish. The chances are, therefore, 

 that in any one womb nearly all the embryos will belong to the 

 post-trochophoral or veliger stages; and Erlanger himself once 

 told us that to find material for the adequate study of these 

 earlier stages would require at least two months' search. The 

 opportunities for coming to a decision in the matter are far fewer 

 than in the case of an ordinary Gastropod, whose eggs are laid by 

 thousands, and where every desired stage can be had in abundance. 



int 



FIG. 248. Horizontal section through the visceral hump of an older embryo of Paindinn 

 vivipdra than that represented in Fig. 247, to show the formation of the kidneys and 

 the heart. (After Drummond. ) 



H, rudiment of heart; hep, liver; int, intestine ; l.k, left kidney; l.per, left pericardial sac; r.k, 

 right kidney; r.per, right pericardial sac; sept, evanescent septum between the pericardial sacs; st, 

 stomach. 



Erlanger's account of the matter agrees in principle with what is 

 known of the development of Annelida with very small eggs, like 

 Eupomatus (Hydro'ides), where the mesodermal cells are budded out 

 from the intestine. But of course the formation of a definite pouch 

 is a far more primitive method of development than that so far 

 described for any Annelid or Mollusc, and it is a somewhat strange 

 thing that this mode of development should be found in Paludina, 

 which cannot be described as a very primitive member of the class to 

 which it belongs. 



There the matter must rest, since Erlanger has been cut off by 

 an untimely death, till some other embryologist has the patience to 

 thoroughly investigate this difficult subject. 



When we reach the stage of the development of the pericardium 



