340 



THE INFLUENCE OF LIVING SUEEOUNDlNGS. 



form of growfcli, otherwise determined by the pressure of the 

 crab's abdomen, and we might expect to find that such false sym- 

 metry would disappear, since the mechanical causes determining 

 the gi-owth of the Pachybdella wouM no longer be able to act in 

 the same way. This anticipation is not, however, justified; in 

 the cut here given the abdomen of a crab is shown which bore 

 on it three such uninvited guests, and although a certain 

 irregularity is plainly perceptible in the form and dissimilarity 

 of the three parasites, the false symmetry is quite normal and 

 well developed in all three. This proves that in this case the 

 false symmetry induced by pressure has already become an here- 



FlG. 89.— A. specimen o( Ca;c/n«s mrrnns, from Heligoland, witli three parasitic specimens 

 o£ Sacculbui carcini. All three, in spite of their irregular gi-owtb, exhibit the false 

 symmetry proper to the genus. 



ditary character of the species; otherwise it must have dis- 

 appeared. Thus in this case, what was originally an abnormal 

 and pathological character seems to have become a normal 

 specific character, transmissible by inheritance, 



A still more wonderful instance of the same kind was long 

 •since described by Count Pourtales. During his dredging 

 expedition in the West Indies he discovered a horny coral 

 (see fig. 90J invariably associated with an Annelid. The worm 

 lives in a tube formed by the abnormal growth — which in this 

 species has become normal— of the slender branches of the coral ; 

 they grow together into a rather fine network, and thus form 



