INTRODUCTORY CHAPTER. 



5 



they are usually provided with tentacula, by 

 which the animals feel their way, and which 

 they have the power of 

 easily retracting when 

 danger threatens ; they 

 have occasionally also 

 eyes, and other organs of sense, but they vary 

 much in this respect, some possessing those of 

 sight and hearing*, whilst others are devoid of 

 both, and apparently enjoy only that of touch. 

 That limb, which, from its being an organ of 

 motion, has been called the foot, is much more 

 universally found amongst mollusca than the 

 head, and is a member of considerable import- 

 ance. A few words only are necessary with 

 regard to the internal construction of these ani- 

 mals ; they possess the usual organs necessary 

 for the support of life, a nervous system, a 

 heart, with its attendant veins and arteries, 

 furnished with a cold and white or blueish 

 blood, and an apparatus for breathing, consisting, 

 in those which live in water, of branchiae placed 

 externally or internally, and so arranged as to 



* The latter sense, it is said, is confined to the Cepha- 

 lopoda, 



B 3 



