NOTES ON BOPYRUS SQUILLAKI i. 277 



they usually infested the female prawn only, for out of several hundreds 

 infested the male prawn was free. 



Dr. Fritz Midler made, in the year L864, a vers remarkable 

 observation on a member of the Bopyridce, which he communicated to 

 the authors of the " British Sessile-Eyed Crustacea." He says : " One 

 of the most interesting animals of this family is a Bopyrus living on 

 Pag urus (a genus of the hermit crab), in which the dorsal surface of 

 the parasite is directed towards the Pagurus. (He therefore named it 

 Bopyrus resupinatus.)" The origin of this curious mode of attachment 

 is the following : — The larva of Bopyrus affixes itself to Sacculina pur- 

 purea (another parasite of the non-segmented suctorial order of 

 crustaceans (Rhizocephala) living on the same Pagurus), and takes its 

 nourishment from the roots of the parasite. After the death of the 

 Sacculina, to whose central surface the Bopyrus was fixed, the latter 

 probably cannot change its position, and remains with its dorsal surface 

 facing the Pagurus. 



Finally, in briefly contrasting together the two adult animals, the 

 host Palcemon and its parasite Bopyrus, we have the symmetrical, 

 compact, segmented body of the one, and the unsymmetrical body of 

 loose consistence of the other — the cephalothorax with its stout 

 rostrum and compound eyes gives place to a mere extension of the 

 body of the other — the complicated mouth of the one is represented by 

 a mere sucking apparatus in the other — the long and sensitise 

 antennas of the one are represented by merely dwarfed extensions in 

 the other ; the ramifying branchiae of the one give place to rudi- 

 mentary organs in the other ; the long, slender, and graceful walking 

 and swimming feet of the one are represented by dwarfed limbs in the 

 other; but, as a compensation, and the only one of the greatest import- 

 ance to the parasite, the hands are both strong and numerous to aid it 

 in grasping and holding on. 



If I have at all succeeded in enabling the members to gain a 

 conception of the relative differences, not only between the typical 

 crustacean as host and its particular parasite under consideration, 

 but also between the different stages of growth of that par:: 

 I think they will agree with me that no better illustration could 

 be adduced of "the effects of use and disuse of parts," and of 

 the " adaptation of the organism to its environment." As Mr. Herbert 

 Spencer has pointed out in the " Principles of Biology " before referred 

 to, animals of the Annulose type become unsymmetrical when their 

 parts are unsymmetrically related to the environment. The common 

 hermit crab (Pagurus) furnishes an illustration like Bopyrus. The 

 embryos of each of these creatures are symmetrical, but the curvature 

 of the body of the hermit crab is due to the position it acquires to 

 adapt itself to the shell which it inhabits, and the unsymmetrical 

 condition of the adult Bopyrus is similarly due to the position it occu- 

 pies within the carapace of its host the prawn. Except for the 

 writings of Dr. Darwin and Mr. Herbert Spencer such biological 

 problems as that presented in the morphology and degraded structure 



