covers with smooth pearly matter to prevent irritation. The beautiful 

 iridescence is caused by light falling on the edges of transparent plates. 

 The nucleus of the pearl may be any foreign substance, like a grain of 

 sand, particle of food, or a parasitic worm. Spherical pearls are frequently 

 found loose in soft parts of pelecypods, particularly in the muscles. 



"' While we are discussing the subject of pearls, let us look at a broken 

 shell of a Umo. If you observe this closely with a hand lens you will 



see that it is composed of three distinct parts. Now, 

 if you will look at this section, which I have prepared 

 for the microscope, you will see that the shell is made 

 up of an outer layer, composed of almost black tissue, 

 a layer of columnar tissue set at right angles to the 

 first layer, and a third or prismatic layer, which forms 

 the inner surface of the shell, and is composed of thin 

 pl^es of membrane and carbonate of lime placed alter- 

 fresh - water clam, na telv. It is the effect of the rays of light falling on 



showing the three : . , . 



shell layers, a, epi- the edges of these transparent plates which gives to 

 layer* c, prismatic, or these shells, and also the pearls which the animals 

 inner layer. produce, their beautiful iridescent luster." 



After finishing the larger clams, we cleaned a number of the smaller 

 shells belonging to the genus Sphcerium. Upon opening some of these 

 clams, we found a number of smaller shells, which were about one-sixth 

 the size of the larger shell. These, the Professor informed us, were the 

 young of the Sphcerium, which are born alive and not hatched from eggs. 

 He also added, that they are very active when young, climbing about 

 water-plants, and seeming to thoroughly enjoy themselves ; and that they 

 frequently suspend themselves by a thread, called a byssus. 



Having prepared and cleaned the bivalves, we turned our attention to 

 the snails. In order to kill the animal, these were treated in the same 

 manner as were the clams, after which the soft parts were extracted 

 with a piece of wire bent into a spiral and sharpened at one end to a 

 fine point. Many of the snails, especially the apple snails, were covered 

 with hard mud which it was necessary to remove. This we accomplished 

 with a tooth-brush. Some of these specimens were badly stained with 

 iron, and although we scrubbed them diligently, the stain would not come 

 out. Noticing our predicament, Professor Parker took a bottle of oxalic 

 acid solution and told us to place them in this for a few moments. Fol- 

 lowing his advice, we found that they were cleaned perfectly. 



He also made each of us a good snail extractor by taking a pair of 

 hat pins and heating them over the gas flame. With a pair of pincers 

 he twisted the pin into a spiral of two turns. Then he heated it again, 



56 



