The royal natural history . is anterior to the heart. The majority of themolluscs 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 ofthe Tectibranchs. They are generally furnished with a pair of tentacles and labialpalpi, 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 shouldbe consulted. It represents a longitudinal section of the animal; p is the foot; a,the mouth, covered above with the veil-like exp
The royal natural history . is anterior to the heart. The majority of themolluscs 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 ofthe Tectibranchs. They are generally furnished with a pair of tentacles and labialpalpi, 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 shouldbe 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 tentaclesc; v shows the branchial veins carrying the blood to the gills, from which it flowsinto the heart. This position is the opposite of that which characterises theProsobranchs. Another anatomical peculiarity, which may here be referred to, isthe direct communication of the system of blood-vessels with the surroundingmedium—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 tothe blood, and through which the blood-vessels permeating the back and foot, likethe holes in a sponge, can be filled or emptied at the will of the animal. Althoughthis, in the main, is the principle of the circulation in most Opisthobranchs,one branch of the order possesses no special breathing-organ, respiration beingeffected through the naked skin of the body. The Opisthobranchia may bedivided into three principal suborders, namely, Nudibranchiata, Tectibranchiata, andPteropoda. Naked-Gilled Subgroup,—Suborder Nudibranchiata. The naked-gilled Gastropods constitute a large assemblage of extremelybeautiful molluscs, of remarkable shape, and often brilliant coloration. Thedistinguishing characteristic of the typical forms consists in the breathing-organ
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