ON CRABS WHICH CLING. 



BY H. J. FLEURE, D.SC, 



University College of Wales, Aberystioyth. 



No student doubts that the ten-legged stalk-eyed hunters 

 ^\e know as lobsters and crabs represent the culminating 

 point in the development of the Crustacean type. They 

 can often swim fairly efficiently and most of them can 

 also walk, in fact they represent all stages in the evolution 

 of an exclusively walking habit from an almost exclusively 

 swimming habit. The simplest and oldest types among them 

 (Pena^ids) only swim, but all the others can do more or less 

 Avalking. The great class of shrimps and prawns keep up 

 both methods of locomotion and so do the lobsters, though, 

 among these, lurking and walking probably take up the greater 

 part of the animal's life. It is at any rate from some 

 members of the lobster group that the walking Decapods 

 have descended. These last present a great diversity of form 

 and adaptation, but a whole group of them {Brachyura) may 

 be considered together as descendants of some type near 

 Dromla, the interesting beast which is so anxious to cover its 

 back with any other animal that may be handy. 



Another group in which the strangest of modifications 

 are apparent is that of the Hermit Crabs, descended from 

 forms like Nitea and Cliulianassa^ long-tailed Decapods 

 deficient in lime. 



There is however still another group which came off from 

 the trunk of the genealogical tree not far from the point 

 whence liromia and the ordinary crabs have sprung. This 

 group includes the two common genera Galathea and Por- 

 cellana, the first being obviously the more lobster-like, i.e,^ 

 the less modified of the two. 



As befits a form which is in a sense near the boundary 

 line between two grou23s we find a notable mixing of habits in 

 the Galatheas. They often lurk in crevices waiting for 

 any prey that may move past, and in correlation with this 

 we notice that the abdomen or tail is much weaker than 

 in those more active swimmers, the lobsters and shrimps. We 



