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 carcinization. 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 

 physicjue 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. 



