COMMENSAL ASSOCIATION. 349 



The conclusion reached by Weymouth as to the cause under- 

 lying the association he reports is entirely plausible, but what is 

 one to say in the case of Stonwlophus and Libinia? It does not 

 seem possible that either commensal was concerned with the food 

 habits of the other ; no crabs were observed feeding on dead speci- 

 mens of the medusa, and though they may do so in deeper waters, 

 this type of crustacean is not known to attack living prey. It is 

 certain that the crab added the burden of freight to the jellyfish, 

 but probably did it no other injury : also any possible benefit to 

 the latter seems unlikely. On the other hand it is unquestionable 

 that the crab received both shelter and transportation, though 

 whether either or both of these benefits accounts for the condition 

 is problematical. That the association has anything to do with the 

 life history of the crab appears doubtful, since all specimens found 

 were adults and long past the delicate stages of Cancer gracilis 

 collected by Weymouth. The only other speculation which occurs 

 to the writer is that the crab might resort to such shelter at the 

 molting periods, but here again fact does not support theory, as 

 those individuals observed all had fully hardened shells. The 

 purpose of this curious union then remains to be determined by 

 more extended and exact studies. 



Another interesting angle of this situation is the problem of 

 how the crab attains the medusa. Since the former is absolutely 

 confined to the bottom in so far as its own efforts are concerned, 

 there remain but two alternatives : either the medusa must descend 

 to the substratum at least occasionally, and for an obscure pur- 

 pose, or else one of the larval stages of the crab must seek shelter 

 within the umbrella and then remain attached during a long period 

 of its mature life, for a reason equally difficult to conjecture. 



And if the facts of the case are not sufficiently strange, consider 

 also the circumstance that this observation has so long escaped 

 scientific detection and is also unknown to all of the local fisher- 

 men, boatmen, and sportsmen whom the writer interviewed, and 

 this in spite of the familiarity of the animals concerned and the 

 well nigh universality of the association in the region covered. 



Spider crabs are famous for their protective adaptations, adorn- 

 ing the carapace with algse, sponges, and hydroids, or possessing a 

 shell so colored and sculptured as to blend effectively with the en- 



