MOLLUSCA. 409 



to, contribute to augment its resources. The sense of sight 

 is by no means universally enjoyed by the inhabitants of 

 this class. In a few species, the eye is constructed on the 

 plan of the same organ in the vertebral animals. In gene- 

 ral, however, it appears only as a black point, the peculiar 

 functions of which can only be inferred from analogy. In 

 many animals there is no trace of an eye, consequently they 

 cannot possess that varied information which the higher 

 animals derive from that organ. Where eyes exist in this 

 class, they are uniformly two in number. In one tribe on- 

 ly, namely the cuttle-fish, the rudiments of the organs of hear- 

 ing have been detected. The organs adapted to smelling 

 cannot be exhibited, but the existence of the sense is de- 

 monstrated by the facility with which they discover suitable 

 food, when placed within their reach. The sense of taste 

 exists, but it is difficult to point out the particular parts of 

 the mouth fitted for its residence. As they, however, select 

 particular articles of food in preference to others, it may be 

 concluded that taste regulates the choice. 



In the classification of the mollusca, the characters fur- 

 nished by the nervous system, from the difficulty of their 

 detection and exhibition, have never come into use. But 

 those furnished by the organs of perception are highly priz- 

 ed. Of these, the eye is the most obvious and constant. 

 It varies in position in different species ; but, among indi- 

 viduals of the same species its characters are constant. 



In the cutaneous, muscular, and nervous systems, traces 

 of a general plan may be observed, according to which they 

 have been constructed in the different tribes. In the or- 

 gans which remain to be considered, there is less uniformi- 

 ty of structure, each family, almost, being constructed ac- 

 cording to a model of its own. 



The time when molluscous animals feed has not been care- 

 fully attended to. Those which Jive in the water are be- 



