364 OF RESPIRATION. 



being filled with water, and then suddenly emptied at tolerably regular 

 intervals. 



654. In the various classes of the Molluscous sub-kingdom, we find 

 the respiration provided for, by the adaptation of distinct organs for the 

 purpose. As most of the animals of this* group are inhabitants of the 

 water, the respiration is usually carried on by means of gills, rather than 

 by any organ resembling a lung. The latter is found, however, in a 

 few species ; such as the Snail, Slug, and other terrestrial air-breathing 

 Molluscs, and usually consists of a simple cavity, situated in the back, 

 communicating directly with the air through an aperture in the skin, 

 and having its walls covered with a minute network of blood-vessels. 

 The form and position of the gills differ extremely in the several classes 

 of Molluscous animals. In the lowest the respiratory surface is formed, 

 as in the higher Polypes, by a dilatation of the Pharynx ; but sometimes, 

 instead of surrounding a large cavity, it forms a special riband-like fold 

 of membrane, passing from one end of it to the other, on which the 

 blood is minutely distributed. In this group of animals, there is a 

 regular system of canals for the conveyance of the blood ; but these, in 

 many parts of the system 4 , and especially on the respiratory membrane, 

 do not seem to be furnished with distinct walls, and are rather mere 

 channels excavated in the tissues. And the circulation is liable to a 

 continual change in its direction, the blood being sometimes transmitted 

 to the respiratory surface before it proceeds to the body, and sometimes 

 after it has traversed the other tissues ( 557). The water in contact 

 with the respiratory surface is continually renewed by the action of the 

 cilia, with which it is thickly covered. 



655. In certain of the Molluscs, inhabiting bivalve shells, we find 

 that the internal surface of the mantle or skin that lines the valves, is 

 the special organ of respiration ; the external water having free access 

 to these by the separation of the skin along the edges of the valve, so 

 that it enters the cavity in which the viscera are lodged, and bathes 

 their exterior. But in most bivalve molluscs, the internal surface of 

 the mantle is doubled (as it were) into four riband-like folds, which are 

 delicately fringed at their edges, and which have, in fact the same 

 essential structure as the gills of higher animals ( 663). To these the 

 blood is transmitted, when it has been rendered venous by traversing 

 the vessels of the body generally : and in these it is exposed, through 

 a surface which is greatly extended by the minute division of the fringes, 

 to the action of water introduced from without, and constantly renewed 

 by ciliary action. In many of these animals, as in the common Oyster, 

 the two lobes of the mantle are so completely separated, that the water 

 can still enter freely between the valves, but in general, they are more 

 or less united, so that the cavity in which the gills lie is partially closed. 

 There is always a provision, however, for the free access of water from 

 without by means of two apertures, one for its entrance and the other 

 for its ejection, and in certain species which burrow deeply in sand or 

 mud, these apertures are furnished with long tubes, or siphons, which 

 convey the water from near the entrance of the burrow, and carry it 

 thither again. In these also, a continual flow of water over the respi- 



