GENERAL CONSIDERATION. 251 



more or less developed, though still closely pressed against the body. 

 This stage, which replaces the Mysis stage in the Anomura and 

 Brachyura, has been called by Claus (No. 7) the Metazoaea. 



6. Last stages of metamorphosis. These stages are now only 

 distinguished from the adult form to which they lead by unim- 

 portant characteristics. In the Sergestidae, the loss of the exopodite 

 on the thoracic limbs and the enlargement of the abdomen leads 

 from the Mysis stage to the Mastigopus stage (Fig. 120 C, p. 262). 

 In the Penaeidae and Caridea the corresponding stage is called 

 the first shrimp stage. The last stages of metamorphosis in the 

 Anomura and Brachyura are known as the Megalopa (Fig. 133 ^4 

 and B, p. 287), the transition from this stage to the adult form 

 involving considerably greater changes in the Brachyura than in 

 the Anomura, because the latter remain throughout life at a stage 

 of development nearer that of the Megalopa. 



If we review the series of larval stages here described, we see 

 that the order of development of the segments and limbs from before 

 backward is as a rule retained. Only in details are there certain 

 characteristic deviations from this order. For instance, the develop- 

 ment of the thoracic segments in the Zoaea stage is usually retarded 

 as compared with the development of the abdominal segments, and, 

 among the limbs, an exception occurs in the early appearance of the 

 sixth pair of pleopoda. These exceptions to the rule can be shown 

 to result from the adaptation of the larva to a pelagic existence. 



The whole series of ontogenetic stages described above is only 

 passed through in very few cases in the Decapoda. Penaeus and 

 Lucifer may serve as examples of such complete metamorphosis. As 

 a rule, metamorphosis is more or less abbreviated, the early stages 

 being obscured or hurried through within the egg. The Sergestidae, 

 for instance, hatch at the Protozoaea stage, most of the Caridea 

 at the Zoaea stage, the marine Astacidea at the Mysis stage. The 

 most extreme case of abbreviated metamorphosis is found in many 

 forms living in fresh water and on land (Astacus, Telphusa, and 

 Gecarcinus). 



The abbreviation of metamorphosis is also attained in another 

 way, viz., by the tendency towards the obliteration of the charac- 

 teristics of the special ontogenetic stages. Thus we shall see that, 

 in the Caridea, the Zoaea stage appears altered by anticipating 

 certain of the peculiarities of the Mysis stage. The complete dis- 

 appearance of the Mysis stage in the metamorphosis of the Brachyura 

 and Anomura can be explained in a similar way. 



