MOLLUSC A 301 



nc outside shell. This includes the Pearly Nautilus, the Octopus, and Cuttle- 

 fish, and forms the highest class of molluscs, and, indeed, of Invertebrates 

 generally. 



Class 2. Gasteropods (literally, " belly-footed "), which have, for the most 

 part, single shells (Univalves), as compared with the double shells (Bivalves) 

 of Class 4. Most of them have a " foot," for purposes of locomotion, and are 

 marine animals, though others are found in fresh water, and, like the snail, 

 on land. 



There is a distinct sub-class known as Pteropods (" wing-footed "), some- 

 times called a separate class. They have two lobes of the foot, divided into 

 " wings," or wing-like fins, by which they swim. They live in immense shoals 

 in the open sea, and are the prey of many fishes, birds, and cetaceans. 



Class 3. The Scaphopoda. This is a very small class, and need not be dealt 

 with here. The Tooth or Tusk shellfish (so-named from the resemblance 

 of the shell to a very small elephant's tusk) Dentalium entale is probably 

 the only member whose shell is likely to be found by young people off the 

 British coast. It is very smooth and white. The Grooved-tusk (D. tarentinum) 

 is delicately grooved at the larger end, and has a pinkish tinge. 



Class 4. Acephalous Molluscs the Lamelli branchiata, or Bivalves such as 

 Mussels, Oysters, and many others, which have two shells hinged together. 

 It should be borne in mind that these have no toothed-ribbons or rasps. 



It is worth while to spend a few moments in observing the formation of 

 shells and how they grow, as hereby we shall grasp the reason for some of the 

 peculiarities we meet with in making a collection of shells. If you examine 

 an Oyster, Mussel, or other bivalve, you will see first of all in opening it that 

 there are two thin flaps, almost transparent, which enclose the animal. These 

 are the " mantle." On the other hand, the mantle of a snail or a periwinkle 

 is a single piece, out of which the head emerges as from a sort of tube ; from 

 which also the animal pushes its " foot," and the horny lid (operculum) which 

 closes the shell when the animal is at rest. (Not all univalves, however, possess 

 this curious lid or door ; and therefore it is of importance in making a collection 

 that the species which do possess it should be preserved with the operculum 

 intact.) 



Notice, next, that many bivalves have shells of unequal size and depth, 

 the deeper one being in all cases the lower the cradle, as it were, of its occu- 

 pant ; the shallower one serves as a lid. 



When very young these shellfish are minute, and the shell is like a glassy 

 head. As the animal grows this shell must, of course, increase hi size propor- 

 tionately. The animal extends its mantle till it reaches over the edge of the 

 shell, and exudes a very thin fluid, which, being very limy, forms a new layer, 

 and, in fact, a new rim to the shell. This process is repeated when necessary, 

 and so the shell grows. 



In many cases it is not difficult to trace the " lines of growth." The Garden 

 Snail, for example, shows them very plainly on its shell. 



