MORPHOLOGY OF LAMELLIBRANCHIATE MOLLUSKS. 407 



In Niicida and Area tlie veutricle lias tlie 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 (Ko, 12) regards it as the primitive condition, pointing to a 

 double origin of the heart. Grobben (Ko. G) 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 (ISTo. 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 tliat Ziegler (No. 24) has 

 shown the jjericardium of Cyclas to be formed in development fi-om 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 " 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 x^erforated heart was formed by a fusion 

 of the two parts of its ventricle below the rectum. In Oatrea, 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 sj)ace of My a arenaria (Fig. 27, ^y) is quite small, while in 

 Venus mercenaria (Fig. 16,^) it reaches a very great size. Lacaze-Duthiers showed 

 that the heart of Anomia did not lie in a pericardium, aud he believed that none 

 existed. Pelseneer, however, thinks that he has found the remains of the pericardial 

 sj)ace 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 ijossession 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 aortse. 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 fi'om the upi)er (morphologically anterior) end of the ventricle and 



