141 



CHAPTEK XIX. 



MOLLUSC A. 



The Flustra or Sea-mats. Avicularia. Metamorphoses of the Flustra. Salpae 

 and Ascidise. Botrylli. Pyrosomata. Bivalve Shell-fish. Free and Sessile. 

 The Byssus. The File of the Pholades. Eespiration of the Bivalve Shell- 

 Fish. Their Nourishment, Snails. Their Masticatory Apparatus. Their 

 Cautious Habits. Pteropods. Conical Appendages of the Clio's Head. Its 

 wonderful Dental Apparatus. Cuttle-Fish. Sucking-Disks. The Onychoteu- 

 this. Number and Importance of the Molluscs. 



THE naked slug lazily crawling in damp weather over our 

 garden and forest-paths, the oyster firmly attached to the 

 bottom of the sea, the many-armed cuttle-fish rapidly darting 

 through the waters, and the ponderous whelk slowly dragging 

 along its heavy shell in the deeper waters beyond the recess of 

 the tide, are no doubt very dissimilar in their outward form and 

 in their mode of life; but on examining their internal structure 

 a close relationship becomes apparent, and thus they are all 

 comprised by naturalists under the vast class of the molluscs 

 which spreads in countless legions over the surface of the globe, 

 but chiefly inhabits the seas ; as of the living species, whose 

 number is estimated at 16,732, more than 10,000 are denizens 

 of the ocean. 



The molluscs are distinguished from all other animals by the 

 following characters : Their bodies are soft, but almost all of 

 them have a peculiar development of the skin which covers 

 their body like a mantle, and generally secretes a hard, inar- 

 ticulated, and consequently inflexible, calcareous or horny shell. 

 Their blood is white, flows from the heart to all parts of the 

 body, and finds its way back again to that organ after hav- 

 ing been refreshed either in lungs, or, more frequently, in a 

 branchial apparatus. Their muscles are attached to various 

 points of their skin, forming more or less dense and complex 



