450 CRUSTACEA MALACOSTRACA. 



nauplius stage of its allies fully developed, but with the ab- 

 domen still incompletely segmented. The paired eyes have here 

 begun to appear, as lateral outgrowths of the head. 



The zoaea, already described, ushers in the life- history of most 

 Caridea, the Anomura and most Brachyura. 



The Mysis stage (which would perhaps be more correctly 

 called the Euphausia stage, cf. p. 454) is the initial phase of 

 the marine Macrura Reptantia (Fig. 327), and follows the zoaea 

 stage in the Thalassinidea among the Anomura. The full 

 equipment of thoracic legs is attained, and they bear flagellar 

 exopodites. The abdominal limbs develop during this stage. 



The remarkable pelagic Phyllosoma larva (" Glass Crab ") of 

 the Loricata must be regarded as a highly modified form of the 

 mysis stage (Fig. 326). In contrast with the heavily built 

 adult forms the larvae are delicate, glassy, leaf-like organisms 

 expanded in a horizontal plane, with a narrow constriction 

 between head and thorax, a rudimentary abdomen in the earlier 

 stages, and long, very slender biramous thoracic legs. 



The fresh-w r ater Macrura Reptantia (the Potamobiinae and 

 Parastacinae) are hatched almost in the adult condition, though 

 the young of the former the crayfishes of the N. Hemisphere 

 pass through a stage in which they are said to resemble the 

 latter the crayfishes of the S. Hemisphere. 



The mysis stage does not appear in the life history of the 

 remaining Anomura, or of the Brachyura, in which the transition 

 to the adult condition occurs direct (in the latter with inter- 

 mediate metazoaea and megalopa stages, Fig. 330) from the 

 zoaea. 



In the following table the groups of the Decapod Crustacea 

 are arranged so that we pass, on the whole, from generalized to 

 specialized forms, and the main larval stages are, in each case, 

 indicated by dashes.* The specialization is indicated, among 

 other features, by differentiation in the series of thoracic append- 

 ages, reduction of the trunk limbs (see p. 522) from a biramous 

 to a uniramous type, the differentiation of a branchial chamber 

 beneath the branchiostegite, the coalescence of the neuromeres 



* With regard to the Nephropsidae and Scyllaridae, it is not intended 

 to imply that both or either of these families is directly intermediate 

 between the Caridea and Anomura. They stand here as representatives 

 of the type of the Macrurous Reptantia from which it seems probable 

 that the Anomura are derived. 



