1905.] BEARING ACTINIANS IN THEIR CLAWS. 509 
reach, passing them towards the middle of the disc, from which, 
however, they are abstracted by the ambulatory limbs of the crab. 
Enemies to the crab, too large to be held by the tentacles of the 
polyps, may nevertheless be warned off by the stinging-cells of 
the anemone emitted on irritation. A careful consideration of 
all the cireumstances justifies the view that the crab will secure 
much of its food through the activity of the anemones, and, 
further, that the latter will exercise a protective influence upon 
the crab against larger enemies. The advantages to the actinian 
appear largely negative. As Mobius suggests, the movements of 
the crab will serve to bring the actinian into the neighbourhood 
of more prey, but its chances of ultimately appropriating to 
itself much of this seem very small. The feeding experiments 
demonstrated very clearly that it is only rarely that the actinians 
succeed in ingesting their food ere it is withdrawn by the crab. 
In the case of the actinians Sagartia and Adamsia, commensal 
with hermit crabs, it is usually considered that the polyps secure 
fragments of the food torn up by the masticatory appendages and 
slipping away, but it is not likely that this occurs with Melia. 
Independently of the actinians, the crab can only obtain such 
food as may be lying upon the sea-floor and incidentally come 
upon the maxillipeds and the ambulatory limbs. 
The acquisition of such a peculiar commensal habit on the part 
of two wholly distinct types of crabs, Melia and Polydectus, cor- 
related, in the case of the former at least, with a diminutive size 
and partial loss of activity on the part of the chelipeds, does not 
admit of ready explanation. Among the activities of other 
crustacea there appear to be no examples which help us to 
understand how such behaviour and structural peculiarities have 
become established—no simpler or intermediate stages which 
suggest the lines along which the evolution has taken place. 
In the well-known instances of masking-crabs (Stenorhynchus, 
Dromia) we have the tearing away of suitable objects, such as 
zoophytes, alge, and sponges, which are then affixed to the shell ; 
but the instinctive processes involved therein are less complex 
than in the cases under consideration. In the latter the ordinary 
aggressive and tearing functions of the chelipeds are replaced by 
those of merely holding a living example of another group of 
organisms. Even the seizure by a crab of an anemone and the 
affixation of it upon a gastropod shell, as in the well-known 
hermit crabs Pagurus, and the actinians Sagartia parasitica and 
Adamsia palliata, involves much less of a departure from the 
usual activities of crustacea. 
As in so many morphological and physiological phenomena in 
nature where intermediate stages are not forthcoming, it is 
difficult to see how such an instinct could have been acquired 
or evolved by slow degrees. For instance, while holding the 
actinians the crab could not at the same time employ its claws 
for the usual purpose of seizing and conveying food to its mouth. 
One is constrained to think of mutation as a possible explanation 
