212 



CHAPTERS ON EVOLUTION. 



Again, speaking of the Crayfish, Huxley says : " In this Crustacean, in 

 fact, it would appear that the process of development has undergone 

 its maximum of abbreviation." 



As already remarked, the progressive advance and evolution 

 of a group must naturally include in their course, changes ' and 

 modifications in development as part and parcel of the higher 

 order and structure to which the advancing members of the group 

 attain. It is not surprising, therefore, to find that the crayfish or 

 lobster (Fig. 129, , c) should evince an absence in their develop- 

 ment of those phases and repetitions of their ancestry, of which 

 their lower and more primitive neighbours, the barnacles, &c., pre- 

 sent such typical examples. Whilst, at the same time, it is equally 

 notable and interesting to discover that in Nature's process oft- 

 repeated exceptions prove the rule ; and that here and there, the 

 exceptions to the ordinary development of higher Crustaceans 

 certainly prove that their original way of evolution has lain through 

 the pathways so plainly marked out in the lower ranks of the class. 

 Such exceptions occur within the family circle of the crayfish and 

 lobster kind ; and are even represented in the early history of that 

 most familiar of Crustaceans, the common crab itself. This animal 

 possesses a life-history, which, whilst it presents striking analogies to 

 that of lower Crustaceans, likewise offers some interesting points of 



difference from the develop- 

 ment of the latter animals. 

 Within the egg, as in the 

 case of the crayfish, the 

 youthful crab appears to 

 pass its Nauplius-stage, and 

 sooner or later it emerges 

 upon the world of waters in 

 a form with which our pre- 

 vious researches have not 

 made us familiar. The 

 young crab (Fig. 130, a) 

 possesses a short body, 

 which at first sight appears 

 like a huge head, and a 

 jointed tail. In front and 

 above, are spinous projections, the upper of which reminds one 

 of the end of a nightcap long drawn out. A single and simple 

 eye is placed between two very large compound organs of sight ; 

 four antennae or feelers exist, and three pairs of jaws are developed 

 the young crab thus presenting us with the complete furnishings of 

 the head of the adult. There likewise exist traces of appendages 

 which represent foot-jaws in the full-grown crab, but the jointed tail 



FIG. 130. DEVELOPMENT OF CRAB. 



