570 



Marvels of the Universe 



Photo ill'] 

 THE TEETH OF 



Fron 



Bivalve shells 



A MARINE SNAIL 



Cape Verde Island. 



[/. E. Cooper. 

 iTRITON). 



(e.g., mussels and oysters) 



and fringed on their outer edge, and in colour 

 are a brilliant yellow. On the toothings of the 

 edge are situated the polyps, which are white 

 and semi-transparent. When the tentacles of 

 the polyps are displayed the margin of each 

 wing is irradiated with a series of frail silvery 

 stars. 



SNAILS' TEETH 



BY J. E. COOPER. 



Teeth are usually associated with biting, but in one 

 natural sub-kingdom of animals it is exceptional to 

 find them used for that purpose. The creatures 

 referred to are scientifically called Mollusca — a 

 term which includes Slugs, Snails, Whelks, Squids, 

 Limpets and other shell-bearing animals, 

 have no teeth ; but practically all univalve shells 



possess them, generally in great numbers. They are employed for various purposes, some of which 

 are here described. 



In Slugs and Snails the " tongue," or lingual ribbon, is a slipper-shaped membrane, lying at the 

 back of the throat. Its use is to break up the animal's food, for which purpose it is covered with 

 a multitude of minute pointed teeth composed of chitin, a very hard and brittle substance. The 

 arrangement of the teeth on the ribbon resembles that of the scales on an ancient coat-of-mail ; 

 the teeth themselves can be erected or depressed by special muscles at the will of the animal. The 

 process of digestion in a Snail has been aptly compared to harrowing a field, the food being gradually 

 broken up by the hundreds of minute rasp-like teeth with which the tongue is covered. 



The Great Black Slug of our fields has about thirty thousand of these scale-hke teeth, while the 

 Common Snail manages to rub along with as few as one thousand five hundred. 



In the Glassy Snails the individual teeth are considerably larger and fewer in number, though 

 they serve the same purpose of pulping the food. The small Fresh-water Limpet, which abounds 



under stones in many of our streams, is related to 

 the Snails ; its teeth, however, are more loosely 

 arranged on the tongue and are much narrower in 

 proportion to their size. 



In the majority of the carnivorous marine 

 molluscs both the purpose and arrangement of the 

 tongue are totalh? different from the foregoing. 

 Here the tongue, which is long and narrow, is 

 usually placed in a retractile proboscis. The teeth 

 are fewer but more powerful, and are specially 

 adapted for use as a drill to bore holes in other 

 shells, chiefly helpless bivalves. When once the 

 hole has been drilled the proboscis comes into play 

 as a pump to suck out the victim's juices. 



All the Whelk tribe feed in this manner. From 

 the commercial point of view the Whelks are of 

 some importance. They all have a liking for 

 mussels and oysters, when they can get them. Our 



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Photo by'] 





[/. E. Cooper. 



THE 



CROW-OYSTER-S 



TEETH. 



They are qa 



ile harmless despite 

 appearance. 



their formidable 



