488 THE ANIMAL KINGDOM. 



heart lies in a pericardium in front of the posterior adductor muscle, and 

 very generally beneath the shell-ligament. The ventricle is single, except 

 in Area, cf. Plate vii. Fig. 3, p. 291. There are two auricles, usually separate 

 but united medianly in Ostreidae. The ventricle gives origin to an anterior 

 and posterior aorta. Fine vessels with an endothelial lining are found 

 in the labial tentacles and the intestine. Otherwise the blood spaces are 

 irregular spaces between the viscera and in the mantle. A muscular valve 

 surrounds the large venous channel which brings the blood from the foot 

 to the median infra-cardiac sinus, and by its contraction causes the foot 

 to swell as it does in locomotion. A portion of the mantle appears to 

 form a blood reservoir. The blood is colourless, and contains amoeboid 

 corpuscles. Haemoglobin coloured corpuscles occur in Solen (Ceratisoleti) 

 legumen and Area Noe. The admixture of water with the blood and 

 the presence of pori aquiferi on the margins of the foot appear to 

 be very doubtful. The coloured corpuscles of Solen do not escape even 

 when the animal is strongly irritated and consequently contracted. * The 

 pericardial gland ' (see p. 129) 'appears in two forms, (i) as lobes upon the 

 auricles, (2) as caecal outgrowths of pericardial epithelium into the mantle 

 at the anterior angles of the pericardium. The first form commencing 

 with Area leads through Mytilus and Pecten to Ostrea? where it is de- 

 generate, 'the second commences with Unio and leads to Venus and 

 Scrobicularia ' (Grobben). Venus has also rudiments of the gland on the 

 auricles, probably the more primitive form. Concretions are usually to be 

 found in the epithelial cells, which are sometimes ciliated as in Area and 

 Mytilus, but not in Unio, &c. The first form resembles that found in 

 Cephalopoda \ 



Each ctenidium or gill consists of a longitudinal axis, united for the 

 greater part of its length to the walls of the body but free at its posterior 

 extremity. It contains an afferent and efferent blood-channel, the former 

 bringing blood from the nephridia through which it passes from the median 

 infra-cardiac sinus, the latter conveying it back to the auricles. The 

 ctenidial axis bears two rows of gill-filaments. These are simple and 

 lamellate in Nucula and Yoldia (Arjacea), but are usually more or less 

 tubular, and recurved or bent upon themselves, the outer row externally, 

 the inner row internally. And then either the recurved portion is united 

 to the direct portion by solid interlamellar junctions, and also laterally by 

 interfilamentar ciliary junctions (Area Pectunculus, Mytilus], or by the 

 development of solid interfilamentar junctions, the lattice-like structure 

 of the branchia seen in most Lamellibranchiata is attained. The recurved 

 portions of the outer row of filaments generally unite with the mantle ; 

 of the inner row anteriorly with the body, posteriorly with the corres- 

 ponding row of the opposite side, but they may be either free or united 



1 Grobben, Z. A. ix. 1886, p. 369. 



