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), Ostraca, 

 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 

 Area (A. trapezia, A.pexata, A. tetragona, 

 etc.), in Pedunculus violacescens, Tellina 

 planata (and around the nerve-centres 

 in T. fabula), Pwomya granulata, and 

 some Solenidae such as Ceratisolen 

 legumen. While red in these latter 

 forms, the blood in certain Yeneridae, 

 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 

 part in causing turgescence of tegumen- 

 tary expansions, the mantle and siphons 

 and the foot. 



As in all other Mollusca, the central 

 organ of the circulation is on the dorsal 



ofx 



ncu 



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 ; 

 a.f.r, anterior foot retractor ; cm, 

 auricle; g, internal gill-plate; g', ex- 

 terior gill-plate ; k.a, posterior aclduc- Side (Fig. 202, V6\ near the hinge of 

 tor; hi, hinge -lobe of mantle; A', ,, , 11 j j 



kidney; l.p, labial palp ; pa, mantle ; the Shell, and IS Contained 111 a peri- 

 pe.g, pericardial gland ; p.f.r, posterior 

 foot retractor ; re, rectum ; si, siphon ; 

 ve, ventricle. 



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 



