508 PROF. J. E. DUERDEN ON CRABS [ Dec. WAR 
deopsis shows no important differences, and they may have to be 
regarded as one and the same species. Hence the commensal 
habit may be regarded as not essential to the life of Buno- 
deopsis, and the same can also be said of the Sagartia and 
Phellia. 
Of all actinians, members of the genus Bunodeopsis would 
appear to be the best adapted for the réle of commensalism. 
They are active polyps with long tentacles which are usually 
expanded to their full extent, and in the absence of a sphincter 
muscle the column is incapable of overfolding the tentacles. The 
tentacles are provided with nematocysts of several sizes, and very 
large stinging-cells occur in the spheroidal outgrowths on the 
column; and, lastly, the polyps are easily detached from any 
substratum to which they may be adherent. Were the com- 
mensalism of Melia restricted to Bunodeopsis, its suitability for 
such a relationship is so pronounced as to suggest more than a 
haphazard selection on the part of the crab. But when we 
consider that the same crab will also take up a Sagartia, 1t must 
be admitted that the wisdom of its selection is not so manifest ; 
for this form retracts readily on slight irritation, does not 
re-expand so freely as Bunodeopsis, and is usually very firmly 
attached to its substratum.  Phellia, so far as observations 
upon its activities in aquaria go, seems even less desirable than 
Sagartia for the ccenobiotic habit. 
As regards the dependence of the crabs upon the actinians, the 
case seems much clearer. Owing to the absence of the usual 
junctions of the claws, the commensal habit would appear to be 
necessary to the existence of Melia. The claws, when deprived 
of the anemones, showed no power to grasp or seize other objects, 
not even when food was presented to them. The maxillipeds and 
ambulatory limbs transferred to the mouth any nutritive objects 
offered, but from their non-chelate character these appendages 
can be of little use in seizing or holding prey or warding off 
enemies. There is no question that the procuring of food by the 
crab itself would be very precarious were it not for the assistance 
of the actinians. Moreover, wherever met with, throughout its 
wide distribution, the crab is found to bear actinians. According 
to the observations of Mobius, all the specimens of Melia collected 
by him possessed polyps, though in Borradaile’s experience they 
were sometimes absent from both claws, or from only one. It is 
to be expected that the crabs will occasionally lose their polyps, 
especially during ecdysis, and conceivably they may wander about 
for a time without meeting with others. 
If we attempt to estimate the advantages of the commensalism 
to the two organisms concerned, it must be admitted that the 
result seems to be entirely one-sided, and in favour of the crus- 
tacean. The experiments show that the reflexes of the crab are 
of such a nature as to result in the removal of any food or prey 
which the actinian may secure. The tentacles of the polyp move 
about freely and seize and retain organisms coming within their 
