158 INTEODUCTION TO ZOOLOGT. 



formed from the circumstance, that immense numbers of them 

 passed through a paper iu which holes had been pricked ^^^th 

 a needle of the smallest size." Even without going to foreign 

 countries, or having recourse to the microscope, we have, on 

 cm* own shores, examples of shells remarkable for their 

 minuteness. On one occasion we gathered some handfuls of 

 a small univalve shell {Paludina muriaiica. — Lamarck), which 

 was lying in dark, irregular patches on the strand, near 

 Belfast. It bore considerable resemblance, except in size, to 

 the common fresh- water species (M^. 151). The weight of 

 four quilis, Avhen filled -with these shells, was 

 80 grains; and, as twenty-two of the shells, 

 with their contained animals, weighed only two 

 grains, the number of shells thus enclosed was 

 880. The weight of the quills and their con- 

 tents, when enclosed in a letter, was less than 

 half an ounce; and we were, therefore, enabled 

 to transmit 880 living animals and their habita- 

 Fig. 151. tions fi-om Belfast to Dublm, per mail, for one 

 Palcdina. peuny. 



We have just used the word " habitations," and it is in 

 this light that shells should be -sdewed. They are not beau- 

 tiful productions formed merely to please the eye, but are 

 mansions constructed by molluscous animals for their own 

 especial use and safety. How much is the worth of a shell 

 enhanced in om- eyes by this one consideration! Before, it 

 seemed httle else than a toy, a pretty thmg to look at, and 

 nothing further; but now it assumes an interest in our 

 thoughts; — we ask, how was it fashioned? of what is it 

 composed? whence were the materials derived? by what 

 meaus was it so exquisitely coloured? by what architectural 

 skill was the edifice so contrived that it was adapted, at all 

 periods, to the progressive growth and requkements of its 

 occupant ? 



The shelly matter is secreted by a peculiar organ, termed 

 the " collar" in shells consisting of one piece (univalves), such 

 as the common snail-shell; and by the margins of the 

 cloak or mantle iu those of two pieces (bivalves), such as 

 the oyster or the cockle. The shell was formerly regarded 

 merely as an exudation of calcareous matter, held together by 

 a kind of animal glue. But microscopic observation has 

 shown, that it is a membrane composed of minute cells, dif- 



