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 ( Entoconchci ), 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 
Anthobranchiata. g y mme ^ r f ornij 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 
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 
Doris Tribe. r 1 
time considered forms of the old genus Doris, are now placed in the 
distinct group Doridopsidce, 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 radula, and organised for 
suction. The extensive family Dorididce, 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, 
