MOLLUSCS. 761 



another function, viz. that of introducing water for its admixture 

 with the blood. In the Pteropods, in which HUXLEY 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 1 . In the Heteropods it has the same relations with the 

 branchial cavity and the pericardial sinus 2 ; except that in Firi- 

 loides, which has no respiratory organ, it opens externally on the 

 right side of the body near the vent 3 . In the Ctenobranchiata 

 LEYDIG 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 4 . 

 In the Grymnobranchiata, 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 sinus serves as an auricle 5 ; though by others 

 it is regarded as the kidney 6 .] 



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 [HUXLEY Morphology of Cephalous Mollusca, Phil Trans. 1853, p. 43, GEGEN- 

 BAUER Untersuch. &c. pp. 10 23. 



2 HUXLEY 1. 1. p. 61, GEGENBAUER 1. 1. Atalanta, pp. 121, 123; Carinaria, pp. 

 148, 149, and Zeitsch. f. wissensch., Zool. v. s. 115. 



3 HUXLEY 1. 1. p. 33, LEUCKART Untersuch. in. 1854, pp. 55 57- 



4 LEYDIG Zeitsch. f. Wissensch., Zool. n. pp. 175, 176. PI. xm. fig. 49. 



5 HANCOCK and EMBLETON Anatomy of Doris, Phil. Trans. 1852, pp. 226228. 



6 LEUCKART Zoolog. Untersuch. in. 1854, p. 55.] 



