RESPIRATION. 97 



pidity. This circumstance led Clutms, and some other 

 authors, to think that these protuberances were fins, or instru- 

 ments of motion, and that the animals were fishes. But 

 Reaumur remarked, that they moved these fins with the same 

 rapidity when the animals were at rest, as when they were in 

 motion ; and that, instead of fins, when examined by the mi- 

 croscope, he discovered them to be gills through which the 

 creatures respire. Each gill consists of a short trunk, and 

 two large branches or tubes, which give off on all sides a 

 number of smaller ramifications, and are perfectly similar to 

 the tracheae of other insects. At the origin of every gill, two 

 tracheae penetrate the trunk, and are dispersed through the 

 body of the animal. 



' The Crustacea, the Mollusca, and Worms, all respire by 

 means of gills, which, although they differ in some measure 

 from those of fishes, are formed upon the same plan. In a 

 few instances they respire air by itself, but in general through 

 the medium of water alone. Tn some animals of these classes 

 the gills are situated upon the outside of their bodies, but 

 commonly within. Zoophytes have distinct organs for respi- 

 ration ; yet the air seems, in some way or other, absolutely 

 necessary for their existence also, and probably penetrates 

 their bodies, and acts upon their blood by means entirely un- 

 known. These animals are all cold-blooded. 



' This chapter will be concluded by a few miscellaneous 

 remarks relating to the respiration of different animals, and 

 appearances connected with it. 1 



Land-snails, at the approach of winter, bury themselves in 

 the earth, or retire into holes of rocks, or of old buildings, 

 where they remain in a torpid state during the severity of the 

 season. For protection arid warmth, these animals, when 

 they go into their winter habitations, form, by means of a 

 slime that issues from every pore of their bodies, a membra- 

 nous cover, which stops up the mouths of their shells. But 

 this pellicle or cover, though apparently pretty hard and solid, 

 is so thin and porous as not entirely to exclude the en- 

 trance of air, without which the principle of life could not be 

 continued. Accordingly, when by accident the pellicle is 

 made too thick, and prevents a communication with the ex- 

 ternal air, the animal, to remedy the evil, makes a small aper- 

 ture in its cover. In this state snails remain six or seven 

 months, without food or motion, till the genial warmth of the 

 spring breaks their slumber, and calls forth their active 

 powers. Hence it would appear, tjiat air is more immediately 

 9 



