JOINTED-LIMBED ANIMALS AS MESSMATES 



181 



together in such a fashion that each one assists the other in some 

 definite way, while doing it no manner of harm, they are termed 

 commensals or messmates. For instance, when a hermit-crab and 

 a sea-anemone live together, the hermit-crab, being by nature a 

 very ill-clad and vulnerable animal, acquires by the partnership 

 a thick and easily-adjustable greatcoat, while the sea-anemone, 

 being by nature a hopeless lump of an animal, dependent on 

 chance currents for its 

 food and oxygen, ac- 

 quires an engine and 

 intelligent engine-driver 

 all in one, which are 

 always carrying it in the 

 way of the necessaries 

 of life; and yet with 

 this mutual assistance 

 there goes absolute in- 

 dependence in all other 

 respects, such as mis- 

 tresses and servants, 

 who would both be 

 none the worse for a 

 little knowledge of the 

 principles of zoology, 

 never dreamt of." 



Certain crabs have 

 sponges as messmates, 



th^ mutiiol orlwonf-orr^o Fi g- "29-~ Anderson's Blanket-Crab (CkUenopagurus Andersont] with 

 HUtUai advantages Commensal Sea-Anemones (Epizoanthus) 



being much the same 



as before, it being remembered that sponges are usually avoided 

 by predaceous creatures which appreciate the flavour of crus- 

 taceans. In the members of one family of crabs (Dromidce) the 

 last pair of legs are modified in relation to the commensal habit, 

 being small, with more or less hook-like tips, and having shifted 

 somewhat towards the upper side of the body. They are used 

 to hold a sponge or some other passive messmate that serves 

 as a sort of living cape, promoting concealment and protection. 

 In the common Sponge-Crab (Dromia vulgaris), as the popular 

 name indicates, this companion is a sponge. So also in a little 

 species (Cryptodromia pileifera, fig. 1130) from the coral-reefs 



