398 DEC APOD A. 



The abdomen is reduced to a short imperfectly segmented stump, 

 ending in a fork, between the prongs of which the anus opens. Even the 

 youngest larval Phyllosoma, such as has just been described, cannot be 

 compared with a Zotea, but belongs rather, in the possession of biramous 

 thoracic feet, to a Mysis stage. In the forked tail and Nauplius eye 

 there appear however to be certain very primitive characters carried on to 

 this stage. 



The passage of this young larva to the fully formed Phyllosoma (fig. 

 222 C) is veiy simple. It consists essentially in the fresh development of the 

 first pair of maxillipeds and the two last ambulatory appendages, the growth 

 and segmentation of the abdomen, and the sprouting on it of biramous 

 swimming feet. In the couv/se of these changes the larva becomes a true 

 Decapod in the arrangement and number of its appendages ; and indeed it 

 was united with this group before its larval character was made out. In 

 addition to the appearance of new appendages certain changes take place in 

 those already present. The two posterior maxillipeds, in the Palinurus 

 Phyllosoma at any rate, acquire again an exopodite, and together with the 

 biramous ambulatory feet develop epipodites in the form of gill pouches. 



The mode of passage of the Phyllosoma to the adult is not known, but 

 it can easily be seen from the oldest Phyllosoma forms that the dorsal 

 cephalic plate grows over the thorax, and gives rise to the cephalo-thoracic 

 shield of the adult. 



There are slight structural differences, especially in the antennae, between 

 the Phyllosoma of Scyllarus and that of Palinurus, but the chief difference 

 in development is that the first pair of maxillipeds of the Palinurus embryo, 

 though reduced in the embi-yonic state, does not completely vanish, at any 

 rate till after the free larval state has commenced; and it is doubtful if 

 it does so even then. The freshly hatched Palinnrus Phyllosoma is very 

 considerably more developed than that of Scyllarus. 



Bracliyura. All the Brachyura, with the exception of one or more 

 species of land crabs 1 , leave the egg in the Zotea condition, and though 

 there are slight variations of structure, yet on the whole the Crab Zoasa 

 is a very well marked form. Immediately after leaving the egg (fig. 

 210) it has a somewhat oval shape with a long distinctly-segmented 

 abdomen bent underneath the thorax. The cephalo-thoracic shield 

 covers over the front part of the body, and is prolonged into a long 

 frontal spine pointing forwards, and springing from the region be- 

 tween the two eyes ; a long dorsal spine pointing backwards ; and 

 two lateral spines. 



To the under surface of the body are attached the anterior appen- 

 dages up to the second maxilliped, while the six following pairs of 

 thoracic appendages are either absent or represented only in a very 

 rudimentary form. The abdomen is without appendages. 



The anterior antenna? are single and unjointed, but provided 

 at their extremity with a few olfactory hairs (only two in Carcinus 

 Mcenas) and one or two bristles. The rudiment of the secondary 



1 It has been clearly demonstrated that the majority of land-crabs leave the egg in 

 the Zoaea form. 



