BRITISH NUDIBRANCHIATE MOLLUSCA. 15 



little distance backwards. The testis is composed of two much- branched tubes, which lie one 

 on each side of the body, and extend almost from the head to the tail. The ovary is formed of 

 a number of distinct, globular, ovigerous sacs, which are united by systems of minute tubes to 

 a large central oviduct. This duct, on its way - to the vulva, communicates, as usual, with the 

 testis, spermatheca, and mucus- gland. The apex of the intromittent organ in Alderia carries 

 a crystalline spur. 



Organs of Circulation and Respiration. These consist of central organs of propulsion, — a 

 systemic and portal heart, — arteries, veins, and sinuses or lacunes ; and of laminated, branched, 

 or papillose branchiae, arranged either on the median line, or along the sides of the back. 



The systemic heart lies always immediately below the skin of the back, and consists of 

 two chambers, — an auricle and a ventricle, — enclosed within a pericardium. A large aorta 

 passes forwards, distributing branches to the various organs ; and the blood is returned again 

 to the heart in a partially aerated state, only a portion of it having passed through the 

 branchiae. The blood system is somewhat modified in each of the three families of the order. 

 In the Dorididce, the systemic circulation is twofold, general and partial. By the former, the 

 larger quantity of the blood, after supplying numerous organs, reaches a general system of 

 sinuses or inter-visceral spaces ; then passes into the sinuses or cellular tissue of the skin, and 

 is returned to the auricle by two lateral veins. By the latter, that blood only which goes to 

 the liver-mass, comprising liver, ovary, and kidney, enters the special respiratory organ by a 

 great hepatic or afferent branchial vein, and, after circulating therein, arrives at the auricle 

 from behind by a single, median, efferent or branchio-cardiac vein. 



The partial or hepatic circulation is probably provided with a complete system of capillaries, 

 and has, in connexion with it, a portal circulation, to which is appended a ventricle or portal 

 heart, that lies under the pericardium, the latter being related to the former as an auricle. 

 This portal heart propels venous blood into the renal and hepatic organs. 



In the Tritoniadce, the systemic circulation is likewise divided into two portions ; but in 

 this family, taking Tritonia as the type, there are six afferent branchial, or hepatic veins ; 

 three passing from each side of the liver mass to the branchiae. The blood is returned to 

 the auricle by two lateral venous trunks, or efferent. branchial veins, which, however, do not 

 convey merely aerated blood from the gills, but also drain the blood-sinuses of the skin ; they 

 therefore carry to the heart a partially aerated or mixed stream, and are not to be looked upon 

 as the anatomical equivalent of the efferent branchio-cardiac vein of the Dorididce, but rather, 

 as the homologue of the two lateral veins, bringing the blood from the skin to the auricle. 

 The hepatic circulation,, of the Tritoniadce is undoubtedly as complete as that of the Dorididce, 

 and is also provided with a portal heart. 



The systemic circulation in the Eolididce is simple, there being apparently no partial or 

 hepatic circulation. All the blood sent to the various organs passes into the great abdominal 

 sinuses, or inter-visceral spaces, and then enters the cellular tissue of the skin, a portion of it 

 penetrating the branchial papillae, and in this way it is returned to the heart by a system of 

 efferent branchial veins, which combine to form three great trunks, two being lateral, and one 

 posterior ; these bearing the blood from the gills, also receive it from the sinuses, or cellular 

 tissue of the skin, and consequently pour into the auricle a partially aerated stream, as in the 

 Tritoniadce. The Eolididce are likewise provided with a portal heart, which may probably 

 supply venous blood to the glands of the papillae. 



