224 THE LAMELLIBRANCHIA 
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, Philobrya, 
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 
nee tutes eenevore ection tone’ 8° in. all other Lamellibranehs, 
Kite: iy nceal coemsure:V.auueune? and especially in the siphonate 
liver 3 x, afferent sinus ; XI, retractor muscle of forms, there are always two 
the labial palps; XII, auricle; XIII, ovary; XIV, : 
Tee AUeasex Vievoamniele: 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, Vulsella, 
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. 
Fic. 204. 
