762 “CLASS XIII. 
some (Haliotis, Crepidula, &c.) the rectum, as in most of the bi- 
valves, passes through the heart. Usually the veins of the respi- 
ratory organs are collected in a membranous auricle (atriwm), to 
whose broader part the broad part of the conical ventricle of the heart 
is attached. In most of the Cephalopods three hearts are present ; 
namely, two lateral hearts that drive the venous blood to the gills, 
and a larger arterial heart placed in the middle, that receives the 
veins of the gills. The blood of molluscs is whitish; it is brown, 
sometimes green or red in some Gymnobranchiata, where the colour 
of the body often depends upon that of the blood’. Blood-cor- 
puscles are present in smaller quantity than in vertebrate animals ; 
they are round, in some degree flat and often filled with little 
granules. 
The venous part of the circulating system is always more or less 
imperfect. Cuvier had noticed in Aplysia as early as the beginning 
of this century, that, in place of a vena cava and of branchial arteries, 
spaces are present, which are perforated like meshes and communi- 
eate freely with the cavity for the intestines?. This disposition 
was, however, regarded as an exception. Within the last few 
years it first became apparent, especially from the investigations of 
Minne Epwarps, that a large part of the circulating system in all 
molluscs is formed by interspaces, which surround the different 
organs of the body and are bounded by no special walls®*. The 
venous blood bathes the viscera, and is received into the cavity in 
which these are contained, as into a s¢nus, before being distributed 
to the respiratory organs. The vessels which conduct the blood to 
these organs (the branchial arteries), do not originate in capillaries, 
but have often very conspicuous apertures ; in some the venous sys- 
tem appears to be entirely absent, with the exception of the branchial 
veins which, conveying arterial blood, run to the heart*. In some 
1 KE. Forses in Annals of Nat. History, vi. 1841, p. 317. 
2 Ann. du Mus. 1. pp. 299, 300, Mém. s.1. Moll. No. 9, Pl. m1. fig. 1, G. L. 
figs. 2, 3. 
3 [See note 2, p. 710. | 
4 See Minne Epwarps Ann. des Sc. nat., 3ieme Sér. Tom. 11. Zool. 1845, pp. 
289—315, pp. 341—353, Tom. vit. Zool. 1847, pp. 37—76. In Octopus the visceral 
cavity, in Loligo the cavity surrounding the muscular bulb of the mouth, forms a venous 
sinus. 
