THE LAMELLIBRANCHIA 255 



ganglia, and the otocysts are generally open. The gut may be pro- 

 vided with a relic of the pharyngeal cavity, which in some cases is 

 furnished with two lateral glandular sacs. The auricles of the 

 heart are muscular ; the kidneys are rather simple in structure and 

 glandular throughout their extent. The sexes are separate : the 

 gonads have retained their primitive communications with the initial 

 or internal extremities of the kidneys, but as the two branches of 

 each kidney have acquired a secondary communication at their 

 anterior ends, the genital products pass direct to the external 

 orifice of the kidney by this passage (Fig. 213). 



int afm 



Artult specimen of Yoltlia limatula, represented as seen from the right side, and showing the 

 internal organs, a.a, anterior adductor muscle ; a.f.m, anterior foot muscle ; b.g, byssal gland ; 

 c.g, cerebral ganglion ; e.s, exhalant siphon ; /, foot ; g, gill ; h, heart ; int, intestine ; i.s, in- 

 halant siphon ; l.p, labial palp ; at, otocyst ; p.a, posterior adductor muscle ; pap, palp 

 appendage ; p.e, posterior expansion of the margin of the mantle ; p.f.m, posterior foot muscle ; 

 p.g, pedal ganglion ; s.t, siphonal tentacle ; sto, stomach ; v.g, visceral ganglion. (After Drew.) 



FAMILY 1. SOLENOMYIDAE, Gray. In the gills one row of branchial 

 filaments is directed dorsally and the other ventrally (Fig. 231, g). The 

 mantle has a long postero-ventral suture, and a sjngle posterior orifice. 

 The labial palps of each side are fused together. The shell is elongate ; 

 the hinge has no teeth ; the periostracum is thick. Genus Solenomya, 

 Lamarck. FAMILY 2. NUCULIDAE, Gray. The labial palps free, very 

 broad and provided with a posterior appendage ; all the branchial 

 filaments are oriented transversely ; the shell has an angular dorsal 

 border and the hinge is pliodont ; the mantle is open throughout its 

 extent. Genera Nucula, Lamarck ; the heart situated on the dorsal 

 side of the rectum. Acila, Adams (Cretaceous, Tertiary, and Recent). 

 Pronucula, Hedley. FAMILY 3. LEDIDAE, Adams. The same characters 

 as the Nuculidae, but the mantle has two posterior sutures and two 

 united siphons ; the heart traversed by the rectum. Genera Leda, 

 Schumacher ; the mantle borders produced posteriorly into two lobes 

 which simulate a third siphon. Yoldia, Holier ; siphons elongate ; 



