MOLLUSCA. 13 



Child. It draws its body into its shell for 

 protection. 



Teacher. Yes. The bodies of the mollusca 

 are contractile, that is, have a strong power of con- 

 traction, which they exercise by means of muscles. 

 What more do you remark in these creatures ? 



Child. They have a thick skin, which appears 

 loose in some of them. 



Teacher. This skin is called the sac or man- 

 tie ; it is peculiar to the molluscous tribe, and is 

 constantly moistened by a slimy exudation. It 

 is also full of pores and glands, of which I shall 

 have occasion to speak in a future lesson. Can 

 you explain the nature of pores and glands 1 



Child. Pores are very small holes. I do not 

 exactly know what glands are ; but we have 

 glands in our throats, are those of the mollusca 

 the same as ours ? 



Teacher. They are of the same nature, but 

 much smaller than those in our throats. The 

 office of glands is to secrete or separate fluids of 

 various kinds from the general fluid of the sub- 

 stance. All bodies both of the animal and vege- 

 table kingdom are supplied with a great number 

 of glands ; tears in animals are an example of 

 secretions formed by means of such organs, so are 

 honey and all other peculiar juices of plants. As 

 the glands form little reservoirs, they often ap- 

 pear like small swellings, perceptible to the sight 

 and touch. To return to the mantle ; it some- 

 times envelops the mollusca like a purse, leaving 

 an opening only where the mouth is situated ; in 

 some it extends on the two sides, forming ex- 

 2 



