222 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



this chamber the air is, under the simplest conditions, 

 drawn in by the action of the cilia which cover the 

 surface of the gills or gill plates ; in the Anodon, for 

 example, the currents of water enter into the lower 

 part of the chamber, which in the hinder region of the 

 body is separated from the upper by the union of the 

 gill plates of either side along the middle line ; the 

 water that enters by this lower inhalent passage passes 

 out by the upper or exhalent one. In a number of 

 Lamellibranchs the mantle which bounds these orifices 

 is produced into a more or less long siphon ; these 

 siphons are best developed in forms that burrow in 

 the sand, and which have the siphons directed upwards. 

 A similar kind of gill chamber is formed in many 

 Gastropods by the folding over of the mantle, 

 and in a number of flesh-eating forms a pair of 

 siphons are also developed. The absence of the 

 mantle-fold in such forms as the Nudibranchs 

 leads us, physiologically, to the vague respiration of 

 the gymnosomatous Pteropoda, where the gills have 

 become atrophied. 



The most characteristic organ of the true 

 Cephalopoda is the so-called funnel, which is a 

 modification of part of the foot ; in these highly 

 developed molluscs we have again an example of the 

 relation of the respiratory to the locomotor activity 

 of the animal. When the muscles in the walls of the 

 investing mantle relax they allow water to enter into 

 the gill chamber on either side of this funnel ; when 

 they contract they not only press on the water in the 

 cavity, but also close the orifices by which it entered ; 

 the only passage, then, by which the compressed fluid 

 can escape to the exterior is by way of the funnel, the 

 walls of which, by contracting, aid in the expulsion of 

 the water ; and the final result of this expulsion is, 

 that, unless the Cephalopod is resting on the ground 

 it is driven backwards ; the more often then, water is 



