190 MANUAL OF THE MOLLUSCA. 



The digestive organs of the Nudibranches present two remarkable modi- 

 fications : in Doris and Tritonia the liver is compact and the stomach a sim- 

 ple membranous sac; whilst in JEolis the liver is disintegrated, and its 

 canals so large that the process of digestion must be chiefly carried on in them, 

 and they are regarded as ccecal prolongations of the stomach; the cceca 

 extend into a series of gill-like processes, arranged upon the back of the ani- 

 mal, which also contain part or the whole of the true liver ; the gastric rami- 

 fications vary exceedingly in amount of complexity. 



The vascular system and circulation of the nndibranchiate molluscs is in- 

 complete. In Doris veins can be traced only in the liver and skin; the 

 greater part of the blood from the arteries escapes into the visceral sinus 

 and into a net-work of sinuses in the skin, from which it returns to the 

 auricle by two lateral veins, without having circulated through the gills. 

 The heart is contained in a pericardium to which is attached a small ventricle, 

 or portal heart, for impelling blood to the liver ; the hepatic veins run side 

 by side with the arteries and open into a circular vein, surrounding the vent, 

 and supplying the gills. Only hepatic blood, therefore, circulates through 

 the gills. In JEolis there are no special gills, but the gastro -hepatic papilla? 

 are accompanied by veins which transmit blood to the auricle. The skin acts 

 as an accessory breathing-organ ; it performs the function entirely in the 

 Elysiadte. and in the other families when by accident the branchiae are des- 

 troyed. The water on the gills is renewed by ciliary action. The fry is 

 provided with a transparent, nautiloid shell, closed by an operculum, and 

 swims with a lobed head- veil fringed with cilia, like the young of most other 

 gasteropods. Hancock and Embleton, Phil. Trans. 1852. An. Nat. Hist. 

 1843. 



FAMILY VI. DORID^E.* Sea-lemons. 



Animal oblong ; gills plume-like, placed in a circle on the middle of the 

 back ; tentacles two ; eye-specks immersed, behind the tentacles, not always 

 visible in the adult ; lingual membrane with usually numerous lateral teeth, 

 rachis often edentulous ; stomach simple ; liver compact ; skin strengthened 

 with spicula, more or less definitely arranged. 



DORIS, L. 



Etym. Doris, a sea- nymph. Ex. D. Johnstoni, PL XIII. fig. 1. 



Animal oval, depressed ; mantle large, simple, covering the head and 



foot; dorsal tentacles 2, clavate or conical, lamellated, retractile within 



Mollusca was first clearly demonstrated by M.M. Hancock and Embleton. The 

 excito-motory system of the Mollusca corresponds with the cerebro-spinal system of 

 the vertebrata. 



* Contracted from Dorididce; as the Greeks used Deucalides for Dencaliontiades. 

 Ehrenberg divided the genus Doris into sections, by the number and form of the gills, 

 characters of only specific importance. 



