2i8 CHAPTERS ON EVOLUTION. 



their allies. We have seen that in the crayfish a " Nauplius "-stage is 

 represented ; that in the lobster a Zoea-phase is seen ; that Mysis 

 likewise exhibits a Nauplius, and then settles down as a peculiar 

 form ; that in the crab's early history, a still better marked Zoea 

 appears ; and finally, that the shrimp Penaeus actually passes through 

 a Nauplius phase, a Zoea or water-flea stage, a Mysis form, and finally 

 assumes the likeness of the shrimp tribe. The history of Penseus, 

 therefore, is practically an abridged treatise of the evolution of all higher 

 Crustacea : its development, to parody Pope's line, is " not one, but 

 all Crustaceans' epitome." And as perfectly are the facts of lower 

 crustacean life correlated with those of the higher development of 

 the class. A water-flea, like Cyclops, as an adult, matures its 

 development and ceases to progress at a stage corresponding to 

 that at which Penseus has but attained its. youth. The barnacles 

 and sacculinas exhibit the influence of conditions of parasitism 

 acting at a definite stage in the course of ordinary development, and 

 producing the degraded and attached form of the adults. Mysis 

 advances so far on the way towards the lobster and crayfish type, 

 but stops short in its development at a point represented in lobster 

 history, and beyond which the lobster itself passes as we have seen. 

 Finally, beyond all such stages, and underlying all the variations 

 and obscurities even of the higher and most modified life-histories, 

 we see the Nauplius- form continually appearing as the starting-point 

 of all crustacean history; or as that point, to use Fritz Miiller's 

 expression, which represents the " extreme outpost of the class, 

 retiring furthest into the grey mist of primitive time." The Nauplius 

 appears before us, then, as the founder of the crustacean race. The 

 Zoea is a modification and advance upon the Nauplius ; and from 

 this Zoea (as poved by Penaeus-development) were evolved the 

 higher crustaceans at large. The lobsters and their allies (again 

 appealing to Penaeus) were evolved from the Zoea-form through an 

 intermediate stage represented to-day by the Mysis or opossum- 

 shrimp ; whilst the short-tailed crabs, in all probability, arose directly 

 from the zoea, without the intervention of a Mysis-stage, seeing that 

 in their development they exhibit a distinct Zoea-stage, and do not 

 pass through a Mysis-stage like the lobsters and their long-tailed 

 neighbours. 



Diagrammatically expressed, we may see in the history of crusta- 

 ceans that tree-like arrangement of their pedigree which best 

 illustrates the deductions of evolution. The Nauplius exists at the 

 root of the class. Developed in direct line, we find Penaeus passing 

 through the Zoea and Mysis-stages. The lobster branch diverges 

 after the Mysis-stage has been attained, and the crabs depart from 

 the main stem before the latter phase. The crayfish, with its 

 obliterated Nauplius-stage, may be presumed to have followed the 

 course of development resembling that of the lobster ; its history, 



