550 



PROCEEDINGS OF THE NA TIONAL MUSEUM. 



VOL. XXVIII. 



The carapace, which is at first squarely truncated posteriori}' (fig. 46) 

 and decidedly spindle-shaped, gradually develops a lobe on either side 

 and widens considerably into a form which is obovate or elliptical 

 (fig. 48). The first three segments of the thorax fuse completely with 



the head and the sutures, which 

 subsequently separate the regions 

 of the carapace, appearand become 

 fairly well developed. The free 

 segment of the thorax is still very 

 long in the early chalimus stage, 

 but rapidly shortens with every 

 moult. The genital segment and 

 the abdomen begin to separate by 

 the second chalimus moult, the ab- 

 domen at first being several times 

 the larger (fig. 47). But before 

 the close of the chalimus stage the 

 genital segment has increased until 

 it is the larger, while the abdomen 

 has narrowed somewhat, and the 

 anal papilla? with their seta? have 

 lengthened considerably (fig. 49). 

 The eyes in the early chalimus 

 stages are relatively large and situ- 

 ated far back in the carapace, 

 nearly at its center. They move 

 steadily forward with every moult 

 until they reach the adult position 

 at a point about one-third of the 

 length of the carapace from its 

 anterior edge. But the appendages 

 move forward at the same time so 

 that the eyes remain all the while 

 just above the mouth. 



The first antenna? still remain 

 three-jointed, and in the later chali- 

 mus stages lunules gradually de 

 velop upon the basal joints in such 

 species as are to possess them. This 

 is the final proof that these basal 

 joints become the frontal plates. 

 This fact, which has all along needed the testimony of development 

 in order to demonstrate it fully, now that it is established, entirely 

 changes the homology of the antenna?. So long as the frontal plates 

 were regarded as a portion of the carapace, no homology in structure 

 or function between the antenna? of CaUgus and those of Argidus was 



Fig. 49.— Fully developed chalimus of fe- 

 male Caligus rapax from flounder. Dor- 

 sal view. 



