DORIS. 



common hepatic trunk-vein, lodged in a median depression on the upper surface of the mass. 

 This trunk opens into an internal or venous branchial circle, closely surrounding the anus. 

 The blood thence traverses the branchial leaflets, and falls into a second or external circular 

 canal at the base of the branchial crown ; from the anterior limb of this outer circle, a short 

 wide trunk (m), the true brancho-cardiac vein opens forward on the median line into the 

 posterior border of the auricle of the heart. Here the blood from the liver-mass, having been 

 fully aerated in the special respiratory organ, becomes mixed up with that from the other 

 viscera, which has been partially aerated on its return through the skin to the heart ; the 

 whole surface of the cloak of Boris being covered with vibratile cilia, as well as the branchiae. 

 The surface of the crawling disc is likewise covered with these minute organs. 



The additional propelling organ, situated, as before mentioned, underneath the peri- 

 cardium, is the organ which Cuvier calls " a vesicle acting as reservoir to a canal, which, 

 coming backwards from the liver, opens at the external orifice placed close to the anus." 

 This vesicle or heart (fig. 13 o) is pyriform, and has the interior plicated in a manner to 

 prevent the return of fluid that has once passed through the orifice (q), which is also capable 

 of being contracted like the mouth of a purse. The base or broad end opens into the 

 pericardium, the narrow end tapers to a tube (r), which, after perforating the wall of the 

 large sinus, to be presently described, turns suddenly forwards along the median fissure of 

 the liver, and, branching, goes to inosculate with a minute network formed by twigs of the 

 hepatic arteries. Venous blood from the general visceral cavity, finds its way by minute 

 orifices in the floor of the pericardium into the interior of that cavity, and is thence drawn 

 into the Cuvierian vesicle or heart, and then propelled by it along its tubes and branches, 

 which are arterial in disposition, into the network just described. This apparatus, then, has 

 a decidedly portal character. The blood traversing this network is conveyed through the 

 liver into the hepatic trunk-vein. 



It is evident from the above description, that the systemic circulation is divided into two 

 portions ; one general, the other partial; the latter being combined with a portal circulation. 

 It is only from this hepatic course, in which the blood is more completely deteriorated, that it 

 is sent to the branchiae. 



The small orifice (PI. 2, fig. 1 r, and PI. 1, fig. 13/) near the anus has no communication, 

 as Cuvier thought it had, with this vesicle or portal heart, but leads at once into an extensive 

 more or less ramified cavity or sinus (PI. 1, fig. 13 k, h), the trunk of which extends forwards 

 along the median fissure of the liver. This cavity is circumscribed by a delicate membrane, 

 which is found beneath the pericardium ; it has the aorta in) running along its roof, and the 

 great hepatic vein along its floor, which is intimately adherent to, and undistinguishable from, 

 the capsule of the liver-mass. 



In the wall of the cavity lies the network from the two sources already named ; the 

 inner surface of the whole of this membrane is covered with a fine spongy-looking, glandular 

 tissue, which is most abundant over the tracts of the vessels. It is pretty evident that this 

 is an apparatus for the elaboration of some fluid from the blood, and as the sinus opens 

 externally by a small orifice close to the anus, it may be inferred that it is for excretion, and 

 is probably a renal organ. 



The fine network of veins in the walls of the renal cavity varies in different species. It 

 is most developed in D. Ulamellata (fig. 13), from which the above description is chiefly taken : 



