1 66 



CRUSTACEA EUCARIDA DECAPODA 



soma, which must be regarded as a greatly flattened and modified l 



Mysis stage. 



In the embryo of Palinurus just before hatching (Fig. 112) 



we can recognise the limbs of the head and thorax normally 



developed in order. There are present three thoracic limbs, 



besides the maxillipedes. 

 - wxp. When the Phyllosoma 

 hatches out the first 

 maxillipedes have be- 

 come quite rudimentary, 

 and the second much re- 

 duced, while the second 

 antennae and second 

 maxillae are also re- 

 duced in size. The 

 metamorphosis is com- 

 pleted by the re-develop- 

 ment of the limbs and 



FIG. 113. Phyllosoma larva of Palinurus, sp. x 5. i.u v. 



Ab, Abdomen ; Mxp, 3rd maxillipede ; T, ante- Segments have 



penultimate (6th) thoracic appendage. (After j-, een secondarily SIVD- 

 Cluus.) J r 



pressed during larval 

 life, and by the appearance of the pleopods. 



This process is again met with in the Squillidae (p. 143),' 

 but it resembles the suppression, in so many Decapodan meta- 

 morphoses, of anterior limbs and the precocious development of 

 segments and limbs lying posteriorly. In the ordinary Decapoda, 

 however, the suppressed limbs are merely not formed till later ; 

 while in the Loricata the limbs develop in the correct order, and 

 subsequently degenerate. It is natural to wonder whether the 

 condition of affairs in the Loricata represents the primitive 

 process, and whether the precocious development of segments in 

 the other Decapoda owes its origin to these animals having once 

 had the direct mode of development when the segments were 

 formed in the proper order, and to their having subsequently 

 acquired the larval stages first of all by the degeneration, and 

 then by the suppression of certain segments which were not of 

 use during larval life. The complete metamorphosis, however, of 

 the Peneidea, in which the segments and limbs appear in the 



1 Clans, Unt. z. Erforschung d. genealog. Grundlage d. Crustaceensystcms. 

 Vienna, 1876. 



