136 Affinity of MoUusea and MoIIuscoida. By W. K. Brooks. 
of the embryonic shell ; are bilateral in origin, and together repre- 
sent the dorsal or haemal cup or shell of a Gasteropod, a Polyzoon, 
or a Brachiopod; while the ventral or neural operculum of a 
Gasteropod corresponds to the neural valve of a Brachiopod or the 
lid of a cheilostomatous Polyzoon, and is wanting in the Lamelli- 
branchs. 
The digestive organs of an adult Lamellibranch, although they 
are very much less specialized than those of a Gasteropod, seem to 
be much more widely removed from the embryonic type. The 
stomach of the Yehger of Astyris, like that of a Polyzoon, is divided 
by a constriction into two chambers. (Compare also the figure of 
the embryo of the Pteropod, Carolinia tridentata by H. Pol, and 
that of Limnsea by Eabl.) In the embryo of Mytilus we have, 
according to Lacaze-Duthiers, a similar stomach, and in the adult 
of Yoldia we have the same a little modified ; here the anterior 
portion of the stomach receives the bile-tubes ; and the posterior 
portion is prolonged so as to form a conical, somewhat twisted, 
intestine-like pouch, from the bottom of which the small intestine 
originates. In Yenus 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 anterior is the true intestinal cavity, while 
the posterior contains the crystalline style. In Cardium we find 
the process of differentiation carried a step farther. The partition, 
which in Yenus 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 con- 
tact, and are contained within the same outer wall. Solen will 
answer as an illustration of the next step in the process of dif- 
ferentiation. 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 intes- 
tine 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 
embryo, 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 during the later Yeliger " and the early " Gasteropod " 
