572 



Marvels of the Universe 



The 



sh-water Limpet Kas a remarkably 

 set of teeth. — 



these teeth that it 

 Snail. 



Photos hi/'] 



The Top- 

 Their teeth 

 patterns. 



ihells are a striking contrast to the Volutes, 

 are innumerable, and of three distinct 



largest species is the .\lmond-\vhelk ; some of the 

 smaller kind, however, such as the Dog-whelk, 

 appear to do more damage. At Burnham-on-Crouch 

 the Dog-whelks are collected by the bucketful from 

 the 03'-ster-beds at low spring-tides. 



The common Netted Dog-\Mielk is a more 

 useful member of this tribe. It swarms in crab- 

 pots on our southern coasts, where it plays the 

 part of a scavenger. 



In connection with these carnivorous molluscs 

 it is curious to note how often the attacker does 

 a vast amount of work without reaping any benefit ; 

 it is not at all uncommon to find bivalve shells with 

 several partially-bored holes in them. 



A very degenerate form of the carnivorous type 

 of tongue is found in the Volutes. They possess 

 but one band of trident-shaped teeth, which can- 

 not be of much use to their owner, as the total 

 length of the tongue is three-eighths of an inch, 

 while the shell itself is four and a half inches long. 

 The tongues of marine shells that are vegetarians 

 may be roughly divided into two sections : (i) 

 those in which the tongue is used to collect food ; 

 and (2) those in which it is a digestive organ. 

 Limpets and Periwinkles are good examples of 

 the first section. They have very long tongues 

 armed with comparatively few teeth. The tip of 

 the tongue is protruded when the animal is feeding, 

 and the teeth act as a rake or sickle to scrape off 

 minute seaweeds, etc. As the teeth get worn by 

 • use, new ones are continually being pushed for- 

 ward to replace them. The common Periwinkle 

 can never be short of teeth, as its tongue is twice 

 as long as the shell is high and is said to carry 

 three thousand five hundred teeth. 



In the other section the tongue's chief function 

 appears to be digestive, as in the Snails. Our 

 common Purple Top-shell may be taken as an 

 example. The central teeth are rasp-like and, no 

 doubt, serve to break up the food. The use of the 

 numerous hair-like teeth at each side is uncertain, 

 possibly they act as brushes to push food over the 

 central teeth. 



These remarkable hair-like teeth are also found 

 in our two River Snails, the Viviparas, but in 

 this case they are curved at the extremity. This 

 would make them more effective as brushes. 



The American " Crow-oyster," or " Slipper- 

 hmpet " (which is now well established in some of 



