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). 
Fic. 230. 
Adult specimen of Yoldia limatula, represented as seen from the right side, and showing the 
interna] organs. a.a, anterior adductor muscle ; a.f.m, anterior foot muscle ; b.g, byssal gland ; 
c.g, cerebral ganglion ; e.s, exhalant siphon; f, foot; g, gill; h, heart; int, intestine; 1.8, in- 
halant siphon; /.p, labial palp; of, otocyst; p.a, posterior adductor muscle ; pap, palp 
appendage ; p.e, posterior expansion of the margin of the mantle ; p.fm, posterior foot muscle ; 
p-g, pedal ganglion ; s.t, siphonal tentacle ; sto, stomach ; v.g, visceral ganglion. (After Drew.) 
Famity 1. SoneNomyipag, 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 single 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. Famity 2. Nucunipar, 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. Famity 3. Leprpar, 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, Moller; siphons elongate ; 
