STRUCTURE AND PHYSIOLOGY OF THE MOLLI3SCA. 35 



of the body ; the opening is in front, and occupied by a funnel, 

 which, in the nautilus, closely resembles the siphon of the palti- 

 dina, but has its edges united in the cuttle-fishes. The free 

 edge of the mantle is so adapted that it allows the water to enter 

 the branchial chamber on each side of the funnel ; its muscular 

 walls then contract and force the water through the funnel, an 

 arrangement chiefly subservient to locomotion.* Mr. Bower- 

 bank has observed, that the eledone makes twenty respirations 

 per minute, when resting quietly in a basin of water. 



In most instances, the water on the surface of the gills is 

 changed by ciliary action alone ; in the cephalopods and salpians, 

 it is renewed by the alternate expansion arid contraction of the 

 respiratory chamber, as in the vertebrate animals. 



The respiratory system is of the highest importance in the 

 economy of the mollusca, and its modifications afford most va- 

 luable characters in classification. It will be observed that the 

 Cuvierian classes are based on a variety of particulars, and are 

 very unequal in importance ; but the orders are characterized by 

 their respiratory conditions, and are of much more nearly equal 

 value. 



Orders. Classes. 



Dibranchiata. Owen. 



Tetrabranchiata. Owen. 



Nucleobranchiata. Bl. 

 ENCEPHALA Prosobranchiata. M. Edw. 



Pulmonifera. Cuv. 



Opisthobranchiata. M. Edw. - 



Aporobranchiata. Bl. 

 ( Palliobranchiata. Bl. 

 ACEPHALA j Lamellibranchiata. BL 

 ( Heterobranchiata. Bl. 

 The Shell. The relation of the shell to the breathing-organ 

 is very intimate ; indeed, it may be regarded as a pneumo-skeleton, 



* A very efficient means of locomotion in the slender pointed calamaries, 

 which dart backwards with the recoil, like rockets. 



v CEPHALOPODA. 



GASTEROPODA. 



i 



PTEROPODA. 

 BRACHIOPODA. 

 CONCHIFERA. 

 TUNICATA. 



