GUIDE TO THE MOLLUSCA. 



INTRODUCTION. 



General. 



The Mollusca constitute one of the phyla, or principal divisions 

 of the Animal Kingdom, and include such forms as the Whelk, 

 Snail, Slug, Oyster, and Octopus. They are distributed all over 

 the world and are to be found in the sea, in fresh water, and 

 on land. 



The phylum is very sharply defined and its relationships are 

 obscure ; it would seem to have some distant affinity with the 

 Annelida or segmented worms. 



Although such animals as the Snail, the Oyster, and the 

 Octopus may, at first sight, appear to have little in common, all 

 Mollusca nevertheless resemble one another in the fundamental 

 plan of their structure. They are soft-bodied, unsegmented, 

 invertebrate animals with a muscular projection on the under 

 side, the ' foot ', which serves for locomotion, and with the skin 

 of the back, the ' mantle ', overhanging at the sides as a flap or 

 fold, the space beneath which is known as the mantle -cavity. 

 The mantle nearly always secretes a calcareous shell, and the organs 

 of respiration, whether gills or lung, are usually situated in the 

 mantle-cavity. The Mollusca have a heart and blood-vessels, a 

 complex nervous system, and, in all those that have a definite 

 head, a characteristic rasping organ, the 'radula', within the 

 mouth. 



This definition leaves out of account certain highly specialized 

 and parasitic Mollusca in which the structures mentioned are not 

 found in the adult, although they, or their rudiments, may be 

 present in the young. 



The shell is the most striking feature of most Mollusca, and the 

 various forms it assumes provided a basis for the older classifica- 

 tions of the group. It is in one piece (univalve) and nearly always 

 spirally coiled in the Gastropoda (Snails, Slugs, Whelks, and the 

 like) ; of two pieces (bivalve) in the Lamellibranchia (Oysters, 

 Mussels, Cockles, &c.) ; and concealed within the soft tissues in 

 nearly all existing Cephalopoda (Squids, Octopus). The vast 

 majority of Mollusca belong to one or other of these three 

 great divisions, but a few remain over which are grouped as 



