S4 Natural History of Molluscous Animals: — 



artery [g), whose function is to carry it forwards to the gills 

 or branchiae (/?), where, circulating through the windings of 

 their beautiful leaflets, it is purified, and thence returned by 

 veins running in the reverse direction, and which open at last 

 by a single trunk (z) into the systemic heart, again to run the 

 same endless circuit, 



I have omitted in this description a very remarkable pecu- 

 liarity connected with the venous system, and which merits 

 our particular notice. Previously to their junction with the 

 pulmonic hearts, the two branches into which the great dorsal 

 vein bifurcates, and their accessory veins, pass across two 

 large cavities, called 'venous by Cuvier, which communicate 

 externally by an aperture on each side near the gills. In this 

 part of their course the veins are garnished with some very 

 singular glandular bodies (oc x, Jigs, 25. and 26.), of a spongy 

 cellular structure and yellow colour, from which an opaque 

 yellowish mucous secretion can be easily pressed in consider- 

 able quantity. The cells of these bodies open freely into one 

 another, and they have likewise a very free and direct com- 

 munication with the interior of the veins to which they are 

 appended ( fig. 26.); but of their use it is difficult to form an 



^ opinion. Cuvier 



l ^^K yW^^^^-- :p vVf ^# which the venous 



^^^^^:w!(^® ^^m. ^UJ t)lood is more fully 



^M^IStefiil'%m,. ^^^^^f ^'"^ influence of 

 ^^^^^HSI%''*'^tft • ^ %^ i the circumfluent wa- 



<f ^^x^^^^^^^^^ excretory canals, by 



^^^ glands pour into the 



vein some substance which it could not of itself extract from 

 that fluid ; or, on the contrary, they may be emunctories, by 

 means of which the blood is purged of some noxious principle. 

 This last conjecture, he thinks, is rendered more probable by 

 the abundance of the yellowish mucus poured out; and it 

 is certain that the communication between the interior of. 

 these bodies and the medium in which the animals live is very 

 open ; for when air or an injection is thrown into the vein, the 

 air or the injection passes very readily through the glands 

 into the venous cavity, and thence outwards; or, on the con- 

 trary, if air is blown by its external orifice into this cavity, it 



