Jdne 1, 1899.] 



KNOWLEDGE. 



,139 



ordinary crabs the two remaining pairs of trunk legs take 

 their share in the walking, running or swimming movements 

 for which many crabs are exceptionally well endowed. 

 But in the hermits the two pairs in question are so abruptly 

 shorter than their predecessors, and are so placed that 

 they cannot co-operate in this way. Their services to the 

 organism, however, are none the smaller for their 

 stunted growth. As Dr. C. W. S. Aurivillius has shown, 

 their size, their armature, the plane of motion of their 

 joints, are all convenient adaptations. They are bound to 

 be short and specially folded to suit the conditions of their 

 house-room. The strange-looking pads upon them, when 

 pressed against the smooth sides of the house, supply a 

 fulcrum to enable the animal to draw in and out the heavy 

 claws and trunk. The masses of hairs serve to transfer 

 a glandular secretion from little pits in the carapace to 

 plaster the walls of the dwelling. 



Next we come to the tail part, or pleon. From one 

 point of view this flabby corkscrew of a tail, as seen in 

 common hermits, is very unsatisfactory. It is shockingly 

 unsymmetrical, with some of its appendages entirely 

 wanting on one side. Though belonging to a crustacean, 

 it is to a great extent not crustaceous. This is very un- 

 like a proper crab, the tail of which is practically nothing 

 but crust, and it is equally unlike the tail of a lobster, in 

 which solid meaty muscle is enclosed in a firm, neatly- 

 jointed covering. But it is evident that the hermit, after 

 thinking out the idea of annexing the shell ready-made by 

 a univalve, found it a great convenience to grow a soft and 

 curly tail part, while symmetrical pairs of pleopods or 

 swimming-feet were no longer indispensable when it 

 had given up the practice of swimming. Only in the apex 

 of the tail, that part which in crawfishes and prawns and 

 the like forms the powerful terminal " fan," the hermit 

 had good reason for preserving, not indeed symmetry, but 

 strength of integument. If anything ever was evolved or 

 designed for a special object, one would say that the caudal 

 fan of the JIacrura was designed or evolved to facilitate 

 the motion of the animals possessing it. The hermit crab, 

 by twisting it into the recesses of a univalve shell, has 

 twisted it into an instrument for resisting motion. By 

 means of this so tightly does it cling to its adopted home, 

 that it repels any invitation to leave it as stoutly as Charles 

 the First's little son repelled the hypothetical proffer of a 

 crown, and translates into action the child's emphatic 

 language, " I will be torn in pieces first." 



What now will happen if 

 the obliging mollusc should 

 not be at hand to accommo- 

 date the hermit "? It is not a 

 simple answer that can be 

 given to this question. As 

 Mr. Edward Step has lately 

 observed, a homeless hermit 

 may shelter itself in a niche 

 of a rock. As Aurivillius has 

 shown, various zoophytes 

 come to the assistance of the 

 crustacean, on what may be 

 called reciprocal terms, 

 though we are not just now 

 concerned with the bargain. 

 Most persons know the in- 

 convenience of accommoda- 

 ting an increasing family 

 in a house not made of 

 india-rubber or other elastic 

 substance. They will under- 

 stand, then, the position of a hermit-crab which goes on 



Pylocheles Agassizii A. Milue- 

 Edward?. 



The same in a hollow stone, 

 closing the entrance with its claws. 

 From Huet and Bouvier. 



growing, whereas its home, the shell of an absentee mollusc, 



has lost the power of growth. Where shells are plentiful, a 



change may easily be made from a small one to a larger. 



But in some districts such changes become difBcult or 



impossible. Then it is that the living and growing 



zoophyte, over-spreading the shell, enlarges the borders of 



it for the benefit of the crustacean tenant. Again, several 



members of the Paguridean tribe are exempt from any 



bigoted devotion to con- 

 ohology. There is the 

 S^Bai&BPylochelesAgassizii, 

 which ensconces itself in 

 the hollow of a stone, or 

 of a hard silicious sponge, 

 and, as no twisting of the 

 tail is required for such a 

 residence, the sensible 

 little animal preserves its 

 symmetry. So, too, in the 

 tail-part does Xylopagurus 

 rectus, which lives in a 

 piece of reed or of hollow 

 wood, and of which a 

 peculiar habit has been 



noted or inferred. Pagurids in general, almost of necessity, 



enter then: house tail foremost. But the Xylopagurus is 



widest at the tail-end, and has been found neatly fitted 



into a domicile in which the back entrance is wider than 



the front. As, 



therefore, the 



tail could not 



have passed the 



front doorway, it 



is reasonable to 



conclude that 



the head was in- 

 troduced at the 



postern. Some 



Pagurids neither 



appropriate 



shells nor any 



other ready- 

 made houses. 



Among these is 



the head of the 



tribe, the great 



Birgus latro, 



symmetrical in 



shape, terrestrial 



in habit, fat, and 



well - flavoured. 



It is surnamed 



the Bobber, not 



because it is any 



more a robber 



than the rest of 



come into rivalry 



an acquired taste 



Xylopagiirus rectus A. Milne-Edwarda. 

 Huet and BoiiTier. 



From 



it happens to have 

 universal robber, in 

 These it eats, using 



us, but because 

 with man, the 



for cocoanuts. 

 also the fibres of the husk to line its deep burrow in the 

 ground. Its tree-climbing powers have been disputed ; 

 but on this point we now have Mr. J. Stanley Gardiner's 

 authoritative statement, quoted in Mr. L. A. Borradaile's 

 recent paper (Proc, Zool. Soc.Lond., June 7, 1898): "The 

 robber-crab is very commonly found in the tops, both of 

 Pandanus and of coooanut trees, from which latter I have 

 had it thrown down to me by the natives. It is stated by 

 them to break off the nuts, and often to fall with them, 

 never killing itself, as the coooanut is underneath. I have 

 seen them constantly clinging to the fruit of the Pandanus, 



