GASTROPODS. 
353 
well as the auricle are placed behind the ventricle of the heart, but in the 
Prosobranchs the branchial system is anterior to the heart. The majority of the 
molluscs included in this order are unprovided with a shell in the adult state; 
hut there are some exceptions, such as the shell-bearing Pteropods, and many of 
the Tectibranchs. They are generally furnished with a pair of tentacles and labial 
palpi, or an expansion of the skin like the veil of the larval form. To compre¬ 
hend the character of the internal organisation, the above illustration should 
be consulted. It represents a longitudinal section of the animal; p is the foot; a, 
the mouth, covered above with the veil-like expansion, over which are the tentacles 
c ; v shows the branchial veins carrying the blood to the gills, from which it flows 
into the heart. This position is the opposite of that which characterises the 
Prosobranchs. Another anatomical peculiarity, which may here be referred to, is 
the direct communication of the system of blood-vessels with the surrounding 
medium—a character common to most other molluscs, and on which depends the 
circulation in Pleurobranchus aurantiacus. 
changeable external appearance of the individual. In the illustration of Pleuro¬ 
branchus, as above, g indicates the opening of a duct which conveys water direct to 
the blood, and through which the blood-vessels permeating the back and foot, like 
the holes in a sponge, can be filled or emptied at the will of the animal. Although 
this, in the main, is the principle of the circulation in most Opisthobranchs, 
one branch of the order possesses no special breathing-organ, respiration being 
effected through the naked skin of the body. The Opisthobranchia may be 
divided into three principal suborders, namely, Nudibranchiata, Tectibranchiata, and 
Pteropoda. 
Naked-Gilled Subgroup, —Suborder Nudibranchiata. 
The naked-gilled Gastropods constitute a large assemblage of extremely 
beautiful molluscs, of remarkable shape, and often brilliant coloration. The 
distinguishing characteristic of the typical forms consists in the breathing-organs 
being exposed on the back of the animal, and not protected by the mantle. Other 
groups, however, are classed within this suborder, in which either the position or 
character of the respiratory organs is different. The gills may be situated on 
each side of the body between the back and the foot, or respiration be effected by 
the ciliated surface of the body. For these and other reasons the Nudibranchs 
vol. vi .—23 
