510 ON CRABS BEARING ACTINIANS IN THEIR CLAWS. [DeC. 12, 



of coinmensalism of such a nature, to conceive that a similar 

 instinct has appeared suddenly in the case of two distinct sj)ecies 

 of ciab, and its possession proved favourable to the survival of 

 the individuals. Such an explanation may suffice until it can be 

 put to experimental test, or until extended observations on the 

 activities and structure of animals render the theory of mutation 

 as plausible in the animal kingdom as from De Yries's work it is 

 among plants. 



Summary. 



1. The commensalism between the crab Melia tessellata and 

 actinian polyps is not restricted to a single species of actinians. 

 Of two crabs captured, one carried a Bunodeo2)sis in each claw 

 and the other a Sagartia. 



2. As regards the same crab the two actinian species are 

 interchangeable, and the crabs will dislodge a small polyp of 

 one species to take up a largei- polyp of another (intelligent 

 selection). 



3. Apparently the crab is not aware of the presence of 

 an actinian until it comes into tactile connection with it. 

 Dislodgment of a fixed actinian is brought about by the inser- 

 tion of the first ambulatory limb between the polypal base and 

 the substi'atum. 



4. The crab tiuvels with the actinians expanded and directed 

 forwards, sometimes waving them from side to side. When 

 irritated it responds by moving its chelipeds towards the source 

 of irritation, thereby placing the actinians in what may be con- 

 sidered as the most favourable aggressive or defensive attitudes. 

 The crab reacts in the same manner, whether carrying the 

 actinians or deprived of them. 



5. Food given the polyps is abstracted by the crab by means 

 of its first pair of walking-limbs, the stimulus to activity being 

 derived from the diffusion of the meat juices. 



6. In correlation Avith the commensal habits the crabs have no 

 direct use of the chelij^eds as aggressive or defensive organs, or 

 for grasping objects other than the actinians, and the functions 

 of the first ambulatory appendages are paitly modified. 



7. The commensal actinians present no structural or physio- 

 logical modification compared with closely allied free species 

 elsewhere. 



8. The coenobiotic habit seems to be necessary for the existence 

 of Melia, though not for that of the actinians. 



9. A second species of crab, Polydectus cupilifera, also bears 

 an actinian, Fhellia, in its chelipeds ; specimens of the actinian 

 are also found adherent to stones and coral rock in the natural 

 habitat of the crabs. Thus in all probability a similar commensal 

 habit has been acquired independently by two wholly distinct 

 forms of crabs. 



10. The advantages of the commensalism to the crab are (1) that 



