QQ LAMELLIBRANCHIA. 



consequence of its having developed from an originally single 



rudiment. 



An attempt has been made to explain the rise of a paired heart (as. 

 the original condition) through the relation of the two parts to the 

 -ills lying at the two sides of the body (Thiele, Chap. xxx.). If 

 the paired heart really represents the primitive condition, this ex- 

 planation would be very plausible, but the Annelida, Arthropoda 

 and Amphineura all agree in showing us the heart as an unpaired 

 organ lying dorsally to the intestine. 



We have still to mention that the heart in a few more modified 

 forms {Teredo, Ostrea, Mulleria) lies ventrally to the intestine. In 

 these cases, the union of the pericardial vesicles to form the unpaired 

 heart has no doubt taken place beneath the intestine. [In Nucula, 

 Area and Anomla, the heart is dorsal to the intestine.] 



The condition of the secondary body-cavity and the kidneys in the 

 Lamellibranchia recalls very strikingly those found in the Crustacea 

 and Peripatus (cf. Vol. ii., p. 180, and Vol. hi., p. 204). In these 

 latter, a part of the coelom is directly incorporated in the kidney, with 

 which it is also functionally united. We are perhaps justified in 

 regarding the pericardium of the Lamellibranchia, into which the 

 renal funnel opens as it does in the Arthropoda into the cavities of 

 the primitive segments (or coelom), as the homologate of the end-sae 

 of the excretory organs in these forms. The fact that the coelom- 

 sacs of the two sides are here united, can make no difference, for this 

 does not cause the heart to lie, as may at first appear, in the secondary 

 body-cavity, but it is still found outside that cavity, as it is also in 

 the forms mentioned above. 



If the pericardium possesses the morphological significance ascribed 

 to it. we might perhaps expect that its physiological function should 

 be modified in the same way as in those forms in which the secondary 

 body-cavity has entered into such close relation to the kidney. Tin* 

 assumption seems actually to be confirmed, when we consider the 

 so-called pericardial gland. This gland, the so-called red-brown 

 organ or Keber's organ, arises as outgrowths of the epithelium of the 

 pericardial wall and lies either on the auricles or on the anterior part 

 of the pericardium (Grobben). This organ is most probably ex- 

 cretory and, since it owes its origin to the pericardial epithelium, it 

 seems not unsuitable to ascribe to the latter a similar significance. 

 The close relation in point of position existing between both the 

 pericardium and the pericardial gland and the blood- vascular system., 

 makes such a view appear possible. 



