PHYLUM ARTHROPOD A 305 



(Fig. 227, A). The cephalothorax and abdomen are distinct at 

 this time; eight pairs of appendages are present (I-VIII) and 

 six more are developing (ai-a 6 ). The Zoaa grows and molts and 

 becomes a My sis (Fig. 227, B) with thirteen pairs of appendages 

 (I-VIII) on the cephalothorax. Finally, the Mysis passes into 

 the adult shrimp, which possesses the characteristic number of 

 appendages (I- XIX), each modified to perform its particular 

 function. The Nauplius of Pcnaus resembles the larvae of 

 many simple crustaceans; the Zoaa is somewhat similar to the 

 condition of an adult Cyclops (Fig. 213); the Mysis is like the 

 adult Mysis (Fig. 217); and finally the adult Penaus is more 

 specialized than any of its larval stages, and belongs among the 

 higher Crustacea. The above facts have convinced some 

 zoologists that Penceus recapitulates in its larval development 

 the progress of the race; that the lobster has lost many of these 

 stages, retaining only the Mysis; and that the crayfish hatches 

 in practically the adult condition. The Nauplius stage of the 

 latter is supposed to be represented by a certain embryonic 

 phase (Fig. 207, B). 



The law of biogenesis has been criticized severely by many 

 prominent zoologists, but it has furnished an hypothesis, which 

 has concentrated the attention of scientists upon fundamental 

 embryological processes, and has, therefore, had a great influence 

 upon zoological progress. 



3. Class II. Onychophora 



This class (Gr. onux, a claw; phoreo, I bear) contains about 

 fifty species of a peculiar arthropod, usually placed in a single 

 genus, Peripatus (Fig. 228), but probably belonging to a number 

 of genera. Peripatus has been reported from isolated regions 

 in Africa, Australia, New Zealand, Tasmania, New Britain, 

 Mexico, South America, West Indies, and Malaya, and is, there- 

 fore, an excellent example of an animal with a discontinuous 

 distribution. It lives in crevices of rock, under bark and stones, 

 and in other dark places. As the animal moves slowly from 

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