CHAPTER XV 

 ANIMAL FORMS 



ANIMALS exist in an almost infinite multiplicity 

 IX and complexity of form, and in most cases 

 Jl X. each single kind of animal exists in two or 

 more different — often very widely different — forms. 

 This is familiar to us in the case of the tadpole and 

 the frog, and in the case of the caterpillar, the 

 chrysalis and the butterfly. But these are relatively 

 simple illustrations. In the case of the common 

 eel we have the leptocephalus, the glass-eel, the 

 elver, and finally the eel, while in a group of shrimps 

 known as Pen^us the young are at first a nauplius 

 (cf. fig. IOC, p. 175), then in succession a metanauplius, 

 a protozoasa, a mysis-like creature, and finally an 

 adult, there being in these shrimps no less than five 

 different larval stages in addition to the adult form. 

 In only two of the major groups — the arrow-worms 

 or chastognaths (fig. 62., p. iii) and the rotifers (fig. 

 136, p. 103) — are there no larval forms in any of the 

 included species, the development leading directly 

 from the egg to the adult form. But the same is true 

 in many individual species, or larger or smaller assem- 

 blages of species, in many other groups. In the 

 vertebrates true larval forms leading an independent 

 life occur in most amphibians and a few fishes, 

 but are not found elsewhere. Generally speaking, 

 fresh water representatives of marine animals either 



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