408 



REPORT OF THE COMMISSIONER OF FISHERIES. 



seven pairs of appendages, which are usually designated as the first 

 and second antenna, the mandibles, the first and second maxill;^, and 

 the first and second niaxillipeds, the latter being provided with long 

 plumose hairs and used as the principal organs of propulsion as the 

 animal swims through the water. The other thoracic appendages 

 of the adult crab — namely, the third niaxillipeds and the five pairs of 

 legs — are represented only hy a series of buds lying on each side, 

 almost concealed by the shell. The shell itself is very thin and so 

 transparent that the heart, the intestine, the muscles which move the 

 appendages, and all the other internal organs may be easily observed. 

 The zoa^a sheds its shell a number of times, the bud-like rudiments 

 of the third niaxillipeds and the legs grow a little and the portion of 

 the body which carries them becomes obscurely divided into segments. 



The abdominal feet or 

 swimmerets make their 

 appearance as pairs of 

 buds on the ventral sur- 

 face of the abdominal 

 segmen ts, and certain 

 changes occur in the 

 antennae and mandibles 

 which cause these parts 

 to resemble more closely 

 the parts of the adult crab. 

 For a number of molts 

 the change of the larva is 

 gradual, but after a time 

 it sheds its shell and be- 

 comes suddenl}" converted 

 into a form which is quite 

 different from the zoaea, 

 and which is known as a 

 megalops. The megalops differs from the zorea in the following 

 characters: 



(1) There are no lateral spines and the dorsal spine is very short. 



(2) The eyes are at the ends of very movable stalks. 



(3) The five pairs of legs are full}^ developed and are very similar 

 to those of the adult. 



(4) The gills have made their appearance above the bases of the 

 legs, under the margins of the shell, but these margins are still free. 



(5) The niaxillipeds are no longer organs of locomotion and there 

 are three pairs. 



(6) While the larva is still able to swim, it also moves over the bot- 

 tom by walking upon the tips of its legs, with a crab-like gait, very 



CVT 2.— Megalops form of CalUncctes sapidus or some closely 

 related crab. (After Brooks.) 



