snail Glandina. On the top of the head, the two whip-like tentacles 

 are placed. These have a greater length than that of the whole animal, 

 and are always waving about when the animal is in motion. Just 

 back of the tentacles, the eyes are found at the end of two short, 

 rounded prominences, or peduncles. From the left edge of the aperture, 

 the long, hollow, cylindrical siphon protrudes. This is formed by two 

 extensions of the mantle. On the upper side of the posterior end of 

 the foot, the horny, concentric operculum is placed. When the animal 

 withdraws into its shell, the head first disappears with its appendages 

 and the siphon, then the foot is doubled up in the middle, the oper- 

 culum shutting in last and closing the interior against all enemies. 



" In the farther end of this case you will notice a set of very 

 peculiar shells, some of them looking like marine shells, yet living in 

 fresh water. They live in Lake Tanganyika, a body of water situ- 

 ated in Central Africa, and having a length of four hundred miles 

 and a width of from ten to fifty miles. This lake has an elevation 

 of twenty-seven hundred feet above sea level, and possesses one of the 

 most interesting and peculiar fresh-water molluscan faunas known. It 

 is thought that in some remote period in geological history it formed 

 part of the ocean, and that in the course of time it was cut off from 

 the sea, gradually becoming fresh, and was finally raised to its pres- 

 ent elevation. The reason for such a theory is the presence in the 

 lake of certain molluscan organisms whose shells closely resemble those 

 of the fresh -water family Littorinidce, or periwinkles. The fact that 

 certain species of this family inhabit brackish water, and are even sub- 

 ject to the influence of fresh water, gives additional weight to this 



theory. The shells of one species, Limnotroclms tlio- 

 masi, from Lake Tanganyika, also resemble certain 

 of the top shells ( Trochus) which are marine in hab- 

 itat. Most of the species living in this lake are like 

 the Viviparas in form. 

 Limnotrochus " All of the different groups of the Mollusca have 



tlieir g iants and tneir pig mies > and tlie fresh-water 



mollusks are no exception to the rule. We have, 

 as yet, only studied the animals of normal size, and the giants. Let 

 us now turn our attention to some of the pigmies of these snails. 



" One of the commonest of these small mollusks is the Bytliima tentac- 

 ulata, the shell of which does not exceed half an inch in length, and 

 having the form of a graceful, tapering turret. This species, like many 

 other European animals, has been introduced into America, and bids 

 fair to eclipse many of the native species in the number of individuals. 



22 



