CHAP. X. CLASSIFICATION. 103 



But let us stick to the Crustacea. In Polyphemus 

 Leydig finds the first traces of the intestinal tube even 

 daring segmentation. In Mysis a provisional tail is 

 first formed, and in Ligia a maggot-like larva-skin. 

 The simple median eye appears earlier, and would 

 therefore be more important than the compound paired 

 eyes ; the scale of the antennae in the Prawns would 

 be more important than the flagellum; the inaxilli- 

 pedes of the Decapoda would be more important than 

 the chelae and ambulatory feet, and the anterior six 

 pairs of feet in the Isopoda, than the precisely similarly 

 formed seventh pair ; in the Amphipoda the most im- 

 portant of all organs would be the "micropylar ap- 

 paratus," which disappears without leaving a trace soon 

 after hatching ; in Cyclops the setae of the tail would 

 be more important than all the natatory feet ; in the 

 Cirripedia the posterior antennae, as to which we do not 

 know what becomes of them, would be more important 

 than the cirri, and so forth. The most unimportant of 

 all organs would be the sexual organs, and the most 

 essential peculiarity would consist in colour, which is to 

 be referred back to the ovarian egg. 



" The embryos, or young states of different animals, 

 resemble each other the more, the younger they are" or, as 

 Johannes Miiller expresses it, " they approach the more 

 closely to the common type." Different as may be the 

 ideas connected with the word " type," no one will dis- 

 pute that the typical form of the penultimate pair of 

 feet in the Amphipoda is that of a simple ambulatory 

 foot, and not that of a chela, for the latter occurs in no 



