224 



auricles dilate, so that the three together always fill the pericardial 

 cavity. The walls of the auricles are frequently invested with a 

 brownish-coloured glandular epithelium, constituting the pericardial 

 glands (see below, p. 233). Sometimes the two auricles com- 

 municate with one another inside the pericardium. In the 

 Pectinacea (Pectinidae, Aviculidae) and Ostraeacea (Fig. 203), and 



also in Pectunculus, Philolrya, 

 and the Mytilidae, this com- 

 munication lies behind and 

 to the ventral side of the 

 ventricle and its aorta ; in 

 Isocardia it is anterior and 

 dorsad of the aorta. The 

 same tendency to the union 

 of paired symmetrical organs 

 may be seen in the kidneys 

 and gonads. 



When the circumanal com- 

 plex that is to say, the 

 posterior adductor muscle, the 

 mantle borders, and especially 

 the siphons are only slightly 

 developed, the ventricle only 

 gives off a single anterior 

 aortic trunk, just as it does 

 in the Amphineura and Gas- 

 tropoda. This is the case in 

 the Anomiidae and Mytilidae. 

 Or the posterior aorta may 

 be very small or indistinctly 

 marked off from the anterior 

 aorta, as in Pectunculus and 

 some species of Nucula; but 



n ii _4.i--.. 

 ali Other 



l psnppinllv in tV>* 



, , , , 



iver ; X, afferent sinus ; XI, retractor muscle of f orms. there ai'6 alwaVS tWO 

 he labial palps ; XII, auricle ; XIII, ovary ; XIV, . 1 



FIG. 204. 



Numda nucleus, transverse section through the 

 heart. I, pericardium; II, genital duct; III, ln 

 kidney ; IV, visceral commissure; V, intestine; oi-i 

 VI, foot ; VII, mantle ; VIII, byssus cavity ; IX, a 

 li 

 th , , , . 



rectum ; xv, ventricle. aortae, an anterior and a pos- 



terior, clearly separated from 



one another and of more or less equal importance. The anterior 

 aorta is dorsad and the posterior ventrad of the intestine, except, 

 of course, in Nucula and other forms in which the heart is 

 dorsal. The pedal branch of the anterior aorta passes between 

 the cerebral and pedal ganglion - pairs. In Ostraea, Fulse'lla, 

 Tridacna, and Teredo the two aortae are secondarily fused to form 

 one ; again an instance of convergence in unrelated species, due to 

 the shortening of the antero-posterior axis of the body. 



