MOLLUSCS. 761 
another function, viz. that of introducing water for its admixture 
with the blood. In the Pteropods, in which Huxtey first observed 
it, GEGENBAUER has shewn that this sac with thick spongy walls, 
always seated within the mantle and in the neighbourhood of the 
heart, communicates on the one hand with the respiratory cavity 
by an aperture provided with circular and longitudinal muscular 
fibres, and on the other with the pericardial sinus by a tubu- 
lar opening, which has a sphincter muscle and cilia at its ex- 
tremity’. In the Heteropods it has the same relations with the 
branchial cavity and the pericardial sinus’; except that in Firi- 
loides, which has no respiratory organ, it opens externally on the 
right side of the body near the vent®. In the Ctenobranchiata 
Leypia has shewn in Paludina vivipara, that the triangular kid- 
ney, situated near the heart, and receiving venous blood on its 
passage to the gill, communicates with a wide space which is at 
once its expanded duct and a sac receiving water from the gill- 
cavity by a minute aperture. The water of this sac contains blood- 
globules, so that a direct communication between the water and 
the blood may be presumed to occur in the substance of the kidney’. 
In the Gymnobranchiata, on the other hand, the kidney is de- 
scribed by Hancock and EMBLETON as a branched, tubular, 
spongy organ, inextricably connected with the liver on which it 
lies, and opening by a duct near the vent, but presenting no other 
aperture. It receives, according to these writers, blood partly from 
the aorta and partly from a pulsatory sac which delivers its con- 
tents also to the liver. They regard this sac as a portal heart to 
which the pericardial smus serves as an auricle®; though by others 
it is regarded as the kidney°.] 
The heart is constantly arterial, that is, it receives the veins of 
the respiratory organs and gives off the arteries of the body. In 
1 [HuxteY Morphology of Cephalous Mollusca, Phil. Trans. 1853, p. 43, GEGEN- 
BAUER Untersuch. &c. pp. 20—23. 
2 Huxuey |. 1. p. 61, GeGENnBAUER 1. 1. Atalanta, pp. 121, 123; Carinaria, pp. 
148, 149, and Zeitsch. f. wissensch., Zool, V. 8. 115. 
3 Huxtey |. 1. p. 33, Levcxart Untersuch. 11. 1834, pp. 55—57- 
4 LeypiaG Zeitsch. f. Wissensch., Zool. 1. pp. 175, 176. Pl. xut. fig. 49. 
®> Hancock and EMBLETON Anatomy of Doris, Phil. Trans. 1852, pp. 226—228. 
6 Lruckarr Zoolog. Untersuch. 111. 1854, p. 55. | 
