Xll INTRODUCTION. 



is singularly different from that of its relative among the 

 higher orders of its class. The more important sections 

 are distinguished by the absence, presence, or degree of 

 development of a distinct head, a character correspondent 

 to the degree of the development of the nervous system, 

 and consequent degree of perfection of the organs of sense, 

 and by modifications of the respiratory organs. Most Mol- 

 lusca are furnished with a shell, which is usually composed 

 of a single piece in the higher groups, and is bivalve in the 

 lower. This shell, whether internal or external, may be re- 

 garded as a pneumoskeleton, its principal office being that 

 of serving as a shield, or case, for the protection of the respi- 

 ratory organs, although it often serves as a defensive cover- 

 ing for the whole of the soft parts. The names Testacea^ 

 and " shell-fish,'''' have been bestowed upon the Mollusca on 

 account of their shells, whose elegance of form, richness of 

 sculpture, beauty of colouring and durability of substance 

 have for ages attracted the notice of the curious, and led 

 to their collection and study by lovers of Nature. But 

 whilst the shelly portion of the body of the Mollusk was 

 carefully inspected, treasured, and delineated, the more 

 perishable parts were sadly neglected, and although some 

 of our earlier English naturalists, especially the illustrious 

 Lister, attended to the organization of the entire creature, 

 the shell alone occupied the attention of the majority of 

 observers, even from the time of Linnteus until within com- 

 paratively few years ago. The indefatigable and acute 

 Montagu was indeed inclined to be an honourable excep- 

 tion, and the anatomical studies of Leach led this energetic 

 naturalist to a truer appreciation of the subjects of his 

 researches. The works of Turton, Donovan, and others, 

 frequently cited in the following pages, are monuments of 

 the attention, on the other hand, devoted almost exclu- 



