1876.] 227 [Brooks. 



Venus this peculiarity is much more marked; the posterior chamber 

 is now tubular, and sharply separated from the true stomach, which 

 represents the anterior half of the embryonic stomach. The tube is 

 somewhat convoluted, and is imperfectly divided by a longitudinal 

 fold of the inner wall into two parallel chambers, of which the ante- 

 rior is the true intestinal cavity, while the posterior contains the 

 crystaline style. In Cardium we find the process of differentiation 

 carried a step farther. The partition, which in Venus is imperfect, 

 here extends entirely across the tube, so that the cavity of the sheath 

 of the style is completely shut off from that of the large intestine, 

 although the two are still in contact, and are contained within the 

 same outer wall. Solen will answer as an illustration of the next 

 step in the process of differentiation. Here the large intestine is not 

 united to the sheath of the style, although the former is nearly straight, 

 and parallel to, as well as near the latter. In such forms as Mya the 

 large intestine is entirely independent of the sheath of the style, and 

 its large semicircular convolutions begin at the point where it joins 

 the stomach. This series seems to show that the stomach of a Lamel- 

 libranch is homologous with only the anterior half of that of the em- 

 bryo, or of a Gasteropod, while the large intestine and sheath of the 

 style are together a very peculiar modification of the posterior 

 portion. 



In the prosobranchiate Gasteropoda, as in the Lamellibranchs, the 

 gill is formed as a series of tentacular prolongations into the mantle 

 chamber; these increase in number, and at last form a broad sheet, 

 which is well shown beneath the transparent shell of Crepidula dur- 

 ing the later " Veliger " and the early " Gasteropod " stages. In the 

 Gasteropoda these tentacles remain free from each other during the 

 whole life, and the water circulates over and around them; while in 

 the Lamellibranchs they become so bent upon themselves and united 

 to each other that the gill-tubes are formed, and the water is driven 

 into and through these, to be discharged into the cloaca, which is a 

 special chamber, peculiar to the Lamellibranchs. In such a form as 

 Mytilus, where the union between the tentacles is somewhat imper- 

 fect, we have what appears to be an intermediate stage between the 

 perfect lamella of Mya or Unio and the separate tentacles of a Gas- 

 teropod. The gills of a Lamellibranch are therefore, like the shell 

 and the digestive organs, a specialized form of the embryonic type, 

 which is pretty closely adhered to in the adult Gasteropod. 



These facts must not be regarded as showing that the Lamelli- 



