222 THE LAMELLIBRANCHIA 
the middle line, except in a few fixed forms, such as Pecten (in 
which the anus is to the left of the middle line, Fig. 234, a), Ostraea, 
etc. In some species of Pecten and Lima the rectum is recurrent, 
and nearly completely surrounds the posterior adductor muscle. 
In some special cases, viz. in various Aviculidae, and especially in 
Pinna, the free extremity of the rectum bears an erectile appendage. 
2. Circulatory Apparatus—The vascular system of Lamelli- 
branchs, like that of all other Molluscs, is completely closed, and 
water cannot possibly enter into the circulation. The system is 
composed of more or less dilated and spacious, but none the less 
true vessels, and of sinuses with connective tissue walls, but without 
an endothelium. Not only is the vascular system completely cut 
off from the surrounding medium, but it is also cut off from the 
pericardial cavity, as may clearly be seen in red-blooded forms, 
whose pericardial fluid is colourless and, like that of other Lamelli- 
branchs, totally devoid of blood corpuscles. 
The blood always forms an important part of the mass of the 
body, often constituting a half of its weight. It contains nucleated 
amoeboid corpuscles (amoebocytes), and 
in some cases, particularly in arenicol- 
ous or limicolous species, non-amoeboid 
corpuscles containing haemoglobin. 
Such is the case in various species of 
Arca (A. trapezia, A. pexata, A. tetragona, 
etc.), in Pectunculus violacescens, Tellina 
planata (and around the nerve-centres 
in 7. fabula), Poromya granulata, and 
some Solenidae such as Ceratisolen 
legumen. While red in these latter 
forms, the blood in certain Veuneridae, 
Cardiidae, Dreissensiidae, etc., is of a 
bluish tint owing to the presence of 
haemocyanin. In addition to its normal 
function, the blood plays an important 
Fig. 202. 
Tapes pullaster, without its shell, 
dorsal aspect, with several organs seen 
through the mantle. «a.a, anterior 
adductor muscle; a.b, aortic bulb; 
ajf.r, anterior foot retractor; au, 
auricle; g, internal gill-plate; g’, ex- 
terior gill-plate ; /.a, posterior adduc- 
tor; hi, hinge-lobe of mantle; k, 
kidney ; /.p, labial palp; pa, mantle ; 
pe.g, pericardial gland ; p.f.r, posterior 
foot retractor ; re, rectum ; si, siphon ; 
ve, ventricle. 
part in causing turgescence of tegumen- 
tary expansions, the mantle and siphons 
and the foot. 
As in all other Mollusea, the central 
organ of the circulation is on the dorsal 
side (Fig. 202, ve), near the hinge of 
the shell, and is contained in a peri- 
cardium. In adult Anomiidae, how- 
ever, it projects freely into the pallial 
cavity, behind the adductor muscle. 
The heart always consists of a median ventricle and two generally 
symmetrical auricles: it is only in such forms as Anomia that the 
