PORCELLANOPAGURUS- -BORRADAILE. 
125 
Discussion of the affinities of Porcellanopagurus has brought into view all the 
various crab-like Crustacea. It is not possible to make such a survey without being 
struck, on the one hand, by the persistence with which their habit recurs quite 
independently, and, on the other, by the fact that examples of it are found solely 
upon one branch of the decapod tree. I have elsewhere* shown reason for regarding 
the Anomura and the Brachyura as ultimately forming a single stock of the Reptantia. 
Outside that stock crabs do not occur. Now this fact cannot be attributed to special 
conditions of life. The Anomura are subject to no common conditions which they do 
not share with other Reptantia, and, if conditions of life have induced the origin of 
crabs among Anomura, we are faced with the question why they have not done so 
among other groups of Reptantia or among such reptant Caridea as many Alpheidae 
and Pontoniinae. The habit of body of these Macrura does not, upon the face of 
things, present any greater difficulty to the evolution of something like a crab than 
that of the hermit-crab which gave rise to Lithodes. The conclusion seems inevitable 
that there is in the constitution of the Anomura a disposition or tendency—only the 
vaguest terms can be used here—to achieve that special conformation of body which 
constitutes a crab, and such is not the case with other Decapoda. Whether this 
tendency be primarily one of morphology or of habits is another question ; but seeing 
that a similar form of body has been reached independently in circumstances which 
must have needed very different changes in the habits of the animals, it would appear 
likely that a morphogenetic tendency is the primary factor, but that it can only be 
realized in the event of the development of suitable habits. 
It may be doubted whether the conditions of life play any part other than a purely 
permissive one in the realization of the tendency to carcinizatiou. The circumstances 
in which the life of reptant Decapoda is passed cannot be supposed to have in this 
respect the kind of stringency which dictates, for instance, the special features which 
are common to the pelagic or to the endoparasitic fauna. An incalculable number of 
modes of life is open to them, to be taken advantage of according to the special 
physique of each. The tendency to carcinization, emerging independently from time 
to time, has led in each case to different habits, but the obligation to the change must 
have lain always within, not without the organism. The history of the abandonment 
by hermit-crabs of their habit of living in a shell when they became Lithodidae must 
have been very different from that of the case in which certain Galatheidea, perhaps 
when the broadening of the thorax was permitted by the habit of placing their 
bodies upside down with the flexed abdomen pressed against a stone, became 
Porcellanidae. The true crabs, again, must have arisen in a different manner, 
perhaps when a lobster took to backing into shallow crevices with the abdomen 
doubled under the thorax—a habit which would naturally lead on the one hand 
* The subject of the genealogy of the Reptantia is discussed in the article in Gardiner’s “ Fauna of 
the Maldives,” already quoted above. 
