OF THE ARCTIC MOLLUSC.E. 261 



CLASS.— MOLLUSCS. 



INTRODUCTORY OBSERVATIONS. 



Although, as respects the external configuration of this 

 class, the general plan of their organization is not as uniform 

 as that of the vertebrata, yet there is always an equal de- 

 gree of resemblance between them in their structure and 

 functions. Their nervous system consists of two long cords, 

 running longitudinally through the abdomen, dilated into 

 knots, or ganglia ; the first of these knots is placed over 

 the oesophagus or gullet, and is denominated " brain : " 

 it is scarcely larger than those are along the abdomen, 

 with which they communicate by filaments that encircle 

 the oesophagus like a necklace. The covering or envelope 

 of the body is divided by transverse folds into a certain 

 number of rings, whose teguments are sometimes soft and 

 sometimes hard; the muscles, however, being always situated 

 internally. Articulated limbs are frequently attached to the 

 trunk ; but very often there are none. These animals were 

 named by the late Baron Cuvier " Animalia Articulata" or 

 articulated animals, in which is observed the transition from 

 the circulation in closed vessels to nutrition by imbibition, 

 and the corresponding one of respiration in circumscribed 

 organs, to that effected by trachead or air-vessels distributed 

 throughout the body. In them the organs of taste are the 

 most distinct, one single family alone presenting that of 

 hearing. Their jaws, when they have any, are always 

 lateral.* 



* Cuvier, Regne Animal, vol. i. p. 24. 



