170 



of bone, they are mostly provided with a shell. Mr. 

 Alder, ot Newcastle, has made observations on the 

 pulsations of various mollusca, and he finds in Vitrina 

 it is 120, and in the Nudibranclis 60 to 80. 



In the inland tribes there may be traced a gradual 

 series in the formation and perfection of their shells. 

 Commencing with the Arion, which has separate small 

 granules placed within the mantle of the animal; 

 the Limax, with shelly shield also within the mantle; 

 the Testacella, having a small shell at the tip of the tail; 

 the Vitrina, a shell n<>t large enough for the animal; the 

 Zonites, quite large enough for the animal, but with no 

 thickening around the aperture; the Helix with a well- 

 formed shell reflexed at the mouth; the Cyclostoma,n«tt 

 only thickened, but provided with an operculum ; and the 

 Clausilia with a valve which closes when itretires within 

 the shell. 



Mollusca fall a prey to many hungry creatures ; the 

 water tribe are a food for fish and wild fowl, whilst the 

 land slugs are devoured by the thrush, the Uva of the 

 glow-worm and some beetles. 



Molluscous animals have a heart with arteries and 

 veins for the circulation of their blood, organs for 

 breathing, eyes for seeing, organs for hearing, for 

 smelling, and for tasting, and organs for the digestion 

 of their food. 



Great is the instruction and interest derived from 

 an investigation of specimens of natural history, 

 and to no branch is there more interest attached 

 than to copchology, the elegance and diversity of form, 

 the various habits, and the many species found in and 

 around the Bitish Isles, render this section one of pecu- 

 liar attraction. Where leisure and inclination are 

 combined, a collection made by a personal search 

 proves the most instructive and interesting; but to 

 those not possessing that leisure, or for such species as 



