Crustacea. 



151 



which we employ " diagrammatic drawings " in illustrations of 

 Biology,— to give a general idea of the subject. 



Then comes the complicated yet clearly traceable upward, 

 downivard, and again upward line of evolution. This is clearly 

 manifested in some of the forms here before us, (in the Museum 

 Cabinet) viz., in Lithodes and in Galathea and Porcellana. 



Turning for a moment to this soft bodied and erratic look- 

 ing form, the hermit crab, with its abdomen twisted to one 

 side, and its two hindermost pairs of walking legs reduced to 

 almost useless appendages. We see a case of degradation, a 

 down line, in its evolution, and the answer to the "how ? " in this, 

 is not far to seek. Glancing upon this tray of smaller Macru- 

 rans, in that row representing the fossorial forms, the eye lights 

 upon Callianassa subterranea, a form which burrows in the 

 sand at the sea bottom when it can, but when the bottom is 

 too hard,, it takes shelter in the burrow already made by 

 more powerful excavators. Here is the commencement of a life 

 not quite self-dependent and the result is that Callianassa is 

 not a hard-shelled crustacean. It may be urged " Is it not on 

 account of its being a soft shelled one, that it seeks this 

 shelter ? " The answer to this, to be complete, would require an 

 evening to itself, let it for the moment suffice that the causes 

 and effects act and re-act. Well then, we now see Callianassa, 

 when on the look out for food— the tide covering its lurking 

 place, protruding its anterior half only, from the burrow, this 

 practice brings about that already simply membraneous abdo- 

 men and that weak pair of hinder legs. Now let us glance back 

 " along the files of time " as Dr. Andrew Wilson puts it, and 

 imagine at some remote date a tribe of Callianassse deprived by 

 some means of all soft ground for burrowing, and also of ready 

 made burrows. The next best refuge to be found will be empty 

 univalve shells, and here in the course of generations by the 

 action of the incontrovertible laws of adaptation and of here- 

 dity, the type we now called " Hermit " will be reached. This 

 completes the " down line " I have alluded to. Then during 



