FIDDLER CRAB — HYMAKT. 453 



illipeds are now 9 or 10, but they are not sufficient and the zoea is on 

 the bottom more often than at the top. 



After another week the fourth molt occurs and the fifth zoea 

 (figs. 17 and 18) is formed. This is no longer the graceful, restless, 

 palpitating form of larva that suggested the name Zoea, "Life," to 

 describe it. The body is so heavy that the maxillipeds can only keep 

 it at the surface for short periods. Most of its life is spent drifting 

 along near the bottom. The awkwardness of its movements is in- 

 creased by the greater development of all its primordia. These have 

 no function for the zoea and only serve to impede its movements. 

 In this stage of the larva the antennule shows its first pronounced 

 change. Its basal part becomes swollen and the development of the 

 organ of equilibration, or statocyst, begins within it. (Fig. 19.) The 

 flagellum of the antenna is now elongated and cylindrical. (Fig. 21.) 

 The buds of the thoracic appendages are long and finger-shaped, and 

 at their bases are two or three minute buds that are the primordia of 

 gills. The pleopods are also elongated and finger-shaped. 



For a little over a week the fifth zoea struggles along near the 

 bottom of the sea, driving upward a short distance and then settling 

 down again, and all the time being brushed along by the tide. At 

 length it no longer bobs up at all, but merely rolls along with the 

 tide. All movements have stopped except those of the mouth parts 

 and of the internal organs. The maxillipeds are no longer organs of 

 locomotion. The exopodites are shrunken and partially withdrawn 

 from their sheaths of chitin. Other profound changes in the form 

 of the larva are occurring. The flesh is withdrawn from the great 

 spines of the carapace and from the horns of the telson. The flagellum 

 of the antenna and the distal part of the antennule show dim annula- 

 tions around them. These changes require about 30 hours, and then 

 the zoea undergoes its last molt. It has now been living at sea a 

 little over a month. Of the hundreds hatched only a score or so will 

 have survived. These are rolling helplessly along with the tide when 

 they drag themselves out of their last zoeal shell. 



THE MEGALOPS, A BEAST OF PREY. 



The larva that stretches itself after jerking loose its last attach- 

 ments to the zoeal skin could hardly be recognized as derived from 

 a zoea — even the changed zoea of the fifth stage. As a matter of fact, 

 it was described by earlier naturalists as an entirely separate genus 

 and called Megalops, from its large and prominent eyes. This name 

 has been retained to describe this stage in the larval history, just as 

 pupa describes the second stage in the larval history of the butter- 

 flies. The megalops (figs. 30 and 31) is a larva far different from a 

 pupa, however. Instead of being a motionless, sluggish creature it 

 is a powerfully swimming corsair of the ocean's surface. 



