762 CLASS xin. 



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 (atrium), 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 GymnobrancMata, where the colour 

 of the body often depends upon that of the blood 1 . 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- 

 cate freely with the cavity for the intestines 2 . This disposition 

 was, however, regarded as an exception. Within the last few 

 years it first became apparent, especially from the investigations of 

 MILNE EDWARDS, 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 3 . The 

 venous blood bathes the viscera, and is received into the cavity in 

 which these are contained, as into a sinus, 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 4 . In some 



1 E. FORBES in Annals of Nat. History, vi. 1841, p. 317. 



2 Ann. du Mus. n. pp. 299, 300, M6m. s. I. Moll. No. 9, PL n. fig. i, G. L. 

 figs, i, 3. 



3 [See note 2, p. 710.] 



4 See MILNE EDWARDS Ann. des Sc. not., 3ieme Ser. Tom. in. Zool. 1845, PP- 

 289 315, pp. 341 353, Tom. vm. Zool. 1847, pp. 37 76. In Octopus the visceral 

 cavity, in Loligo the cavity surrounding the muscular bulb of the mouth, forms a venous 



