MORPHOLOGY OF LAMELLIBRANCHIATE MOLLUSKS. 
407 
In JVucula and Mm* the ventricle has the appearance of being double; that is, 
with distinct right and left halves. This has given rise to a discussion as to its mean- 
ing. Milne-Edwards (No. 12) regards it as the primitive condition, pointing to a 
double origin of the heart. Grobben (No. 6) considers the single heart as the prim- 
itive one and believes that this double condition has been brought about by the for- 
ward extension of the foot-retractor or byssus muscles. Theile (No. 22) believes that 
the heart was originally of two independent halves and that upon uniting in the 
median line the heart is found in its various positions, inclosing the rectum or lying 
above or below it. His conclusions are based on the fact that Ziegler (No. 24) has 
shown the pericardium of Cyclas to be formed in development from two symmetrical 
vesicles which unite in the median line. At about the same time that Theile’s paper, 
above referred to, appeared, Pelseneer (No. 17) expressed the opinion that neither the 
views of Milne-Edwards nor those of Grobben were altogether correct. He discards 
the view of the latter entirely. He says that this double condition is not absolutely 
primitive, but is u due to the bilateral separation of the gills (much closer to one 
another in other lamellibranchs) and the auricles.” The primitive heart, he believes, 
was dorsal to the rectum, and the commonly perforated heart was formed by a fusion 
of the two parts of its ventricle below the rectum. In Ostrea , when the heart is 
ventral to the rectum, this position was acquired by the great development of the 
single adductor ; for, as it extended so greatly dorso-ventrally, it carried down the 
gills, and with them the auricles. These, not elongating, also compelled the ventricle 
to move downward. I had myself already come to this conclusion in regard to the 
peculiar position of the heart of Ostrea. 
The pericardium is generally a large space in which the heart lies. Its relative 
size varies extremely, however, in different forms which may be closely related. For 
instance, the pericardial space of Mya arenaria (Fig. 27, p) is quite small, while in 
Venus mercenaria (Fig. 16, p) it reaches a very great size. Lacaze-Duthiers showed 
that the heart of Anomia did not lie in a pericardium, and he believed that none 
existed. Pelseneer, however, thinks that he has found the remains of the pericardial 
space in a flat cavity beneath the rectum, into which open the inner ends of the kid- 
neys. The pericardium is not connected with the vascular system, but into it open 
the excretory organs, one on either side, and, when they are present, the pericardial 
glands. 
In most cases the heart of a lamellibranch gives off an anterior aorta which runs 
forward above the rectum, and a posterior aorta proceeding below the rectum. In 
some forms, however, the posterior aorta does not appear. Such an exception is Sole- 
nomya, one of the most primitive forms. 
It has been claimed that the possession of two aortae was the primitive condition 
and that when the posterior one was absent it had degenerated and disappeared. 
As the general relations of such a form as Solenomya have become known they have 
shown that it is a primitive form; and, as it possesses but one aorta, that condition 
may have been the original one in the group. In the gasteropods there is no pos- 
terior aorta, but a branch of the anterior aorta supplies the posterior part of the body. 
The remaining groups of the Mollusca have both anterior and posterior aortae. Pelse- 
neer (No. 17) considers the single aorta the original condition in lamellibranchs. 
As has been said, the oyster possesses both anterior and posterior aortae. The 
former springs from the upper (morphologically anterior) end of the ventricle and 
