22 A HISTOKY OF EECENT CllUSTACEA 



head and other parts of the whale while the monster is still 

 alive. On the other hand, whales and seals, and fishes 

 large and small, swallow down the Crustacea in a truly- 

 wholesale manner, and so prevent these prodigiously 

 prolific animals from producing a complete block in the 

 cooler parts of the ocean. Independently of this interest- 

 ing exchange of courtesies, which consists in alternately 

 eating and being eaten, there is another kind of associa- 

 tion between crustaceans and other animals, known as 

 commensalism. In this the one creature lives, not at the 

 expense of the other, but merely in companionship with 

 it. Thus, on the common starfish thei'e is found a thread- 

 like minute species of the Caprellidge. Though the star- 

 fish is very frequently to be met with on the shores, its 

 companion Faricumbus typicus (Kroyer) ^ is only seen on 

 dredged specimens, so that apparently this tiny animal 

 has the sense to disengage itself when its host is being 

 driven into an unsuitable or dangerous position. Some of 

 the Amphipoda Hyperidea are very frequently to be found 

 upon jelly-fishes. One of the Gammaridea, Iscea Montagui, 

 Milne-Edwards, appears never to have been found except 

 upon the Spinous Spider Crab, Maia sqidnado, for clinging 

 to which its feet, with their serrate widened extremities, 

 are peculiarly adapted. A French zoologist, M. Edouard 

 Chevreux, some five or six years ago, was searching this 

 interesting crab for its already well-known commensal, 

 when to his surprise, among the algas and hydrozoa, with 

 which the carapace is usually decked, he found not only 

 the species he was in search of, but no less than twenty- 

 two other species of Amphipoda into the bargain. Maia 

 sqidnado is not found far to the northward. On the other 

 hand a very distinct crab, but with some external re- 

 semblance to it, Lithodes maia, is not found far to the 

 southward. Such facts of distribution are often of scien- 

 tific importance. For instance, with regard to the com- 

 paratively narrow strip of land which separates North 

 from South America, the geologist will desire to know how 

 far the crustacean fauna of the sea on one side of the 

 • Formerly Podalirius typicus. 



