354 



MOLLUSCS. 



have been arranged in four groups, Anthobranchiata, Inferobranchiata, Poly- 

 branchiata, and Pellibranchiata. A fifth group, Parasita, has also been proposed for 

 a very curious mollusc (Entoconcha), which lives parasitic within an echinoderm 

 {Synapta). Nudibranchs are found in all parts of the world, and are most 

 abundant in depths where seaweeds and corallines flourish, although they are also 

 found at low water or even between tide - marks. A few species have been 

 dredged at great depths. Some even pass their life in the open sea, attached to 

 floating seaweeds, or swim about freely like Pteropods or Heteropods. 



The animals belonging to this group are characterised by their 

 'symmetrical form, and the position of the vent. This is placed upon 

 the posterior or central part of the back and surrounded by the branching gills, 

 which are more or less contractile. The body is protected above by a large dorsal 

 shield of a spongy texture, and made more or less rigid by calcareous spicules 

 which are embedded in it. The tentacles are dorsal, and retractile through holes 



Anthobranchiata. 



Doris Tribe. 



Acanthodoris pilosa (magnified). 



in the mantle some distance from the anterior extremity of the body, They are 

 smooth at the base, but lamellated towards the tip. The eyes are minute and 

 situated in the skin behind the tentacles. 



A large number of species from all parts of the world, at one 

 time considered forms of the old genus Doris, are now placed in the 

 distinct group Doridopsidxn, on account of differences in the oral organs. The 

 general aspect of the animals of this family is the same as that of Doris, the gills 

 being arranged in a circle around the vent on the posterior part of the back. The 

 buccal mass or eating apparatus is small, without jaws or raf.ula, and organised for 

 suction. The extensive family Dorididw, contains a considerable number of 

 genera and subgenera, characterised by differences in the branchiae, the relative 

 proportion of the mantle in regard to the foot, and variations in the radula and 

 jaws The general aspect of the genus Doris is represented by the figure here 

 given of Acanthodoris pilosa. The species are very numerous, and some of them 

 are amongst the largest of all Nudibranchs. They are world-wide in their dis- 

 tribution, over twenty species occurring in British Seas. "When living, they are 

 most beautiful objects, and must be studied in that state, for when dead and 

 immersed in spirit, they so contract that their natural form can only be guessed, 



