38 NILS HJ. ODHNER, MOLLUSCA. 
Just in the respects named are the most fundamental divergencies be- 
tween the Chamostrea and the Chama gill. In Chamostrea all afferent vessels enter 
into the principal filaments out from the branchial axis, and to the same axis all 
efferent vessels run, debouching into the branchial vein or efferent trunk, which 
carries the blood back again to the heart. As all blood thus emanates from the 
renal sinus, no marginal veins, which are present in the reflected lamella in 
Chama, exist in Chamostrea. A further difference lies in the development of the 
interlamellar septa. These are complete only between the afferent vessels in Cham- 
ostrea, whereas they are high also between the efferent vessels, though not so long, 
and they terminate somewhat sooner, in Chama (cf. fig. 6). 
Fig. 6. Scheme of the intra-branchial blood circuit. Dark venous, light arterial 
interlamellar vessels; some plaits of the exterior lamella and the extension of the 
interior one shown. h. heart; !. p. labial palps; s. r. sinus renalis; 
v. p- vena pallialis. 
As to the minute structure, no differences prevail; in Chamostrea, just as in 
Chama, there are interfilamentar junetions and as a consequence of their existence 
both the anticlinal sides of a fold always contain blood of opposite nature (venous 
in the one, arterial in the other side). 
Being a breathing organ, the purpose of which is to serve the circulatory 
system, the organization of the gill can be wholly understood only in combination 
with the circulatory system, a fact of importance to remember in anatomical in- 
vestigations of lamellibranchs. 
The extra-branchial blood system. The heart of Chamostrea (fig. 5 h.) is 
situated beneath the rectum, from which it is further separated by interjacent ovar- 
ial folliculi. Its ventricle is compressed from the front and behind and extended 
in a transversal direction. From its central part lobes project on each side of the 
