HALF AN HOUR WITH SHELL-FISH. 193 



carnivorous univalves from the herbivorous class 

 exists in the shape of the mouth of the " body- 

 whorl." When it is round or "entire," as in the 

 wentle-trap, it is usually herbivorous ; when notched, 

 or prolonged into a canal, as in the Murex, it is 

 carnivorous. 



It will be as well to devote a few remarks to 

 certain molluscs which, although they have no shells, 

 are classed as gasterapods. They may be found 

 clinging to or lying under rocks and stones at low 

 water ; and many of them compensate for their lack of 

 elegant shells by their brilliant colours. On account 

 of the way in which the gills are exposed, they 

 are called nudilranclis, or " naked-gilled " molluscs. 

 Popularly they go by the names of " sea-lemons," 

 " sea-slugs," " sea-hares," &c. In fact, we may 

 regard them as occupying a similar position in the 

 marine world to that which the common black and 

 yellow garden slugs do on land. Singularly enough, 

 the nudibranch molluscs have an embryonic shell 

 when young, which they lose as they approach 

 the adult condition a fact which throws some light 

 on their descent, and indicates that they may be 

 derived from ancestors which possessed a shell, and 

 were true univalves. The gills, or branchite, are 

 arranged in tufts along the back and sides, hence 

 the name of the order. The nervous system is very 

 highly developed, so much so that some naturalists 

 regard them as holding the highest position among 

 the gasteropods. This is a significant circumstance, 



