THE DECAPODA 



301 



are uniramous. It will thus be seen that the Phyllosoma represents 

 a modification of an early Schizopod-stage. 



A point of some interest in these forms is the occurrence of 

 retrogressive changes in the course of development. Thus the 

 antenna and the maxilla undergo a certain degree of degeneration 

 before hatching, and the seventh and eighth thoracic and the 

 abdominal somites, which are well defined in the embryo, become 

 indistinct in the larva. 



The development of Thalassinidea is interesting on account of 

 the points of resemblance which it shows with the Caridea. The 

 earliest larva is a Zoea, which in some cases (Callianassa and 

 Calocaris) resembles that of Caridea in having the three maxillipeds 



FIG. 178. 



Phyllosoma-larva otPalinurus, just before hatching, ad, abdomen ; L, hepatic caeca ; Il-Vf, 

 thoracic appendages (second and third maxillipeds and first three pairs of legs) ; 1, antennule ; 

 2, antenna. (After Clans, from Korschelt and Holder's Embryology.) 



biramous and natatory ; but in others ( UpogeUa and Jaxea) only the 

 first and second are present on hatching, the third becoming natatory 

 only in the following Schizopod-stage, while the endopodite is still 

 rudimentary, as in other Anomura. The existence of a Schizopod- 

 stage, in which only the last two thoracic limbs are rudimentary 

 and the uropods and rudiments of the pleopods are present, consti- 

 tutes an important distinction from the other Anomura. The larvae 

 of Ja.i'ea ( = Calliaxis) are of exceedingly peculiar form, having the 

 cephalic region produced into a long " neck " resembling at first 

 sight that of Leucifer. To this larval type the name of Trachelifer 

 has been given. The remaining groups of the Anomura and the 

 Brachyura differ from those just described in the suppression of the 

 Schizopod-stage, the legs developing without exopodites in a 

 Metazoea-Bt&ee which follows the Zoea. In the Anomura the Zoea 



