TRITONIA. 



the pericardium, and has the anterior margin produced into a lobe at each side ; it lies, 

 immediately behind the ventricle, and receives at each end a trunk vein (m, m) from the skin. 

 These trunk veins bifurcate within the skin, one branch (n) passing forward, the other (Y) 

 backward close to the margin of the mantle, and drain the blood from the sinuses or cellular 

 tissue of the skin, and from the gills; consequently returning to the heart a stream of 

 imperfectly aerated blood. These trunks are therefore efferent branchial veins. 



The systemic circulation is two-fold as in Boris,— general and partial. By the former 

 all the blood sent to the various organs, except that which is distributed to the liver mass 

 (comprising liver, ovary, and renal organ) must as usual reach the system of inter-visceral 

 sinuses, and so find its way into the cellular tissue of the skin, and from thence to the heart by 

 the lateral or efferent branchial veins just mentioned. By the partial or hepatic circulation, 

 which is probably supplied with a complete set of capillaries, the blood which is transmitted 

 to the liver mass, and that only, is conveyed to the branchiae by six trunk veins (o, o, o) ; three 

 passing from each side of the organ. These trunks are formed by the convergence of 

 numerous branches on the surface of the liver, and each on reaching the skin is divided into 

 two portions (p,_p), one of which passes forward, the other backward : these run along within 

 the skin, immediately below the efferent branchial veins, and give branches to the branchial 

 plumes. It would seem, then, that the branchial circulation is probably as complete as in 

 Doris, the blood being conveyed to and from the gills by a system of afferent and efferent 

 vessels ; but the latter, we have seen, are not distinct, for they carry to the heart also the 

 blood from the skin, which here, as well as in the Doridida and Eolidida, undoubtedly assists 

 in aerating the blood. 



The hepatic circulation has in connexion with it a portal circulation, evinced by the 

 presence of a portal heart (q), which lies as usual below the pericardium, a little to the right, 

 and is related to it as in Doris. This heart is very large, being not much inferior in size to 

 the systemic ventricle ; it is of a cylindrical form, with the ends truncated, and the interior is 

 strongly plicated longitudinally as in Doris. 



The renal organ has not been examined, but its presence is proved by the renal orifice, 

 which is situated close above the anal nipple. It is apparently connected with the liver as 

 the great vessel from the portal heart enters that viscus. 



The cerebral ganglia resemble those of Eolis. The oesophageal are all placed above the 

 alimentary tube ; of these there are apparently at first sight only two principal pairs,— the 

 cerebroid (fig. 9 a, a) and branchial {b, b) being fused so as to form two elongated masses, one 

 on each side the median line, across which they are united by a short commissure. The 

 component parts are, however, distinctly defined by a constriction near the centre of the 

 masses. The olfactory nerves are given off from the upper surface of the anterior portion or 

 cerebroid ganglia. They (1) are long, and, on reaching the dorsal tentacles, each swells out 

 into a rather large roundish ganglion,— the olfactory (d), from the upper aspect of which, 

 numerous filaments pass up the tentacle. A small twig from each of these nerves apparently 

 goes to the veil next the median line. Three pairs of large nerves (2, 3, 4) go from the 

 anterior margin of the same ganglia and supply the veil; one of these (2) sends a large branch 

 to the channel of the mouth ; and another (4) gives a twig to the tentacular point below the 

 veil. Two pairs of small nerves (5, 6) are given off apparently from the commissure, uniting 

 the cerebroid and pedial ganglia ; these go to the skin of the sides of the body. The 



