554 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxviii. 



mandibles, a mouth tube or probocis, first and second maxillipeds, and 

 three pairs of rudimentary swimming legs. 



8. The third moult occurs at about the same interval as the others. 

 The first thoracic segment has been entirely, and the second partially, 

 fused with the head. The third segment has elongated and the rudi- 

 ments of the fourth thoracic legs appear upon its posterior ventral 

 surface. The genital segment still remains fused with the abdomen, 

 but the two have increased considerably in size. The tirst antennas 

 have become three-jointed; the other appendages remain unchanged, 

 save that the swimming legs advance toward the adult -form. 



9. The larva now seeks its host and moults again into what is known 

 as the chalimus stage, the chief characteristic of which is the devel- 

 opment of a long cord or filament by means of which the young are 

 attached to their host. This filament is made out of the material 

 secreted by a median gland situated just in front of the eyes. 



10. The changes which take place during this fourth moult and 

 those which subsequently occur in the several moults of the chalimus 

 period are in the line of gradual development toward the adult form. 

 The carapace, at tirst squarely truncate posteriorly, gradually develops 

 a lobe on either side and widens. The tirst three thoracic segments 

 fuse with the cephalon, and the sutures on the dorsal surface of this 

 cephalo-thorax appear and become well developed. The genital seg- 

 ment and the abdomen separate at the second chalimus moult, the 

 abdomen at first being several times the larger. The eyes move 

 steadily forward with each moult, but as the appendages on the 

 ventral surface move forward also the relation of the two remains 

 unchanged. 



11. The only appendages worth noting are the first and fourth 

 swimming legs. In the early chalimus stages the tirst pair have a 

 well-defined endopod, consisting of a single joint terminating in two 

 small setae. Later this disappears and its place in the adult is indicated, 

 if at all, by a long seta. The fourth legs are made up at first of two 

 short and broad, disk-like segments, without setas or spines. With 

 successive moults they narrow and elongate into the adult form, and 

 acquire both spines and setaa. 



12. This attached stage lasts from four to six weeks, and the larva 

 moults at least five times. At the close of the stage the male is prac- 

 tically fully developed, but the female remains immature, so far as the 

 reproductive organs are concerned, until fertilization has been effected 

 and the ova begin to descend the oviducts. With the last chalimus 

 moult the connecting filament separates just at the frontal margin of 

 the carapace and the copepod enters upon its free adult existence. 



