510 ON CRABS BEARING ACTINIANS IN THEIR CLAWS. [Dec. 12, 
of commensalism of such a nature, to conceive that a similar 
instinct has appeared suddenly in the case of two distinct species 
of crab, 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 Vries’s work it is 
among plants. 
SUMMARY. 
1. The commensalism between the crab Jelia tessellata and 
actinian polyps is not restricted to a single species of actinians. 
Of two crabs captured, one carried a Bunodeopsis in each claw 
and the other a Sagartia. 
9. 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 larger 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 substratum. 
4, The crab travels 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 with the commensal habits the crabs have no 
direct use of the chelipeds as aggressive or defensive organs, or 
for grasping objects other than the actinians, and the functions 
of the first ambulatory appendages are partly modified. 
7. The commensal actinians present no structural or physio- 
logical modification compared with closely allied free species 
elsewhere. 
8. The ceenobiotic 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, Phellia, in its chelipeds; specimens of the actinian 
ave 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 
