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 ; 

 but 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 ix 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. — 2 1 



