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LETTER XV. 



ON THEIR RESPIRATION. 



The respiration of the mollusca is so slow, so little ob- 

 vious, and so easily suspended for a time, that it is possible 

 you may never have observed the process even in those spe- 

 cies which daily cross your path. You will, therefore, in 

 your next walk, please to examine the snail or the slug while 

 they are in progression, and you will see them at intervals 

 open wide a circular hole on the side of the neck, and near 

 the margin of the shield or collar, and, after dilating it to the 

 utmost, they will close it again until its place becomes 

 imperceptible; this they do about four times in a minute, 

 expelling at each time the effete air, and inhaling a fresh 

 supply. In like manner, the aquatic tribes, while crawling 

 along the surface, raise from time to time the pulmonary 

 aperture, in order to emit the vitiated air, sometimes even 

 with a crackling noise, and to receive an equal quantity 

 unadulterated before the aperture is shut. This process is 

 not so obvious in the branchial mollusca, and in many of 

 them, from the position of the gills, such a function is not 

 necessary to renew the water around them. Where, how- 

 ever, the gills are strictly internal, it seems probable that the 

 water is regularly changed when the creatures are in their 

 natural habitats and undisturbed : we know that such is the 

 case with the Cephalopoda, in which inspiration and expira- 

 tion are well marked. " The first (inspiration) is effected 

 by a gradual dilatation of the sac in every direction, but 

 particularly at the sides, accompanied by a subsidence of the 

 lateral valves, collapse of the walls of the funnel, and a rush 

 of water through the lateral openings into the sac. Inspira- 

 tion being completed, the lateral valves are closed, the sac is 

 gradually contracted, the funnel erected and dilated, and the 

 water expelled through it with great force, and in a con- 

 tinued stream." Dr. Coldstream, from whose letter I quote 

 the preceding sentence, has seen the stream emitted by an 

 individual of the Octopus ventricosus, " whose sac measured 

 about four inches in length, carry light bodies to the dis- 

 tance of eleven inches from the orifice of the funnel. Re- 

 spiration is performed more frequently in young than in 



