186 



GENERAL ZOOLOGY 



has been making its way down the coast. It may now be found 

 in southern waters. The new home in America seems to be 

 especially favorable for the periwinkle, for on the rocky coast 

 of New England there are so many of them that on first 

 impression there appears to be no other species. 



The periwinkle depends on plant-food altogether. It is 

 reported that the oystermen of Whitstable, England, at the 

 mouth of the Thames River, collect these snails in large quan- 

 tities and throw them over the oyster-beds in order to be rid 

 of the excess growth of seaweeds. 



On account of its great abundance the periwinkle is much 

 used as an article of human food in England and 011 the 

 Continent. It is one of the many species of snails and slugs 

 used as food, especially by the French people. 



The Nudibranch. The strange-looking creature shown in 

 Fig. 95, Dendrono'tus arbor es'cens, is to be found on seaweeds 



and under submerged 

 rocks along the North 

 Atlantic coast. 



The general form of 

 a slug is, in this animal, 

 modified by the occur- 

 rence of tree-like proc- 

 esses growing on the 

 upper part of the body, 

 whence the generic and 



specific names. These 



Fio.96. Photograph of Living Nudibranch. gseg are oalled 



Natural size 



cer-ata, and to some 



extent are used in respiration. Being thin, oxygen can pass 

 into the animal through them, and the waste carbon dioxide 

 can pass out. The group of animals to which they belong 

 is called Nudibran' chia, in reference to the fact that their 

 gills are naked. There is no indication of a rudimentary 



