340 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



The maxillipeds are much the same as in the 

 preceding stage, except that the exopod of the 

 third maxilliped now has 2 segments. 



The pereiopods liave grown considerably but 

 their structure remains essentially the same. Of 

 the 3 pairs of chelate pereiopods, the third pair is 

 the longest and the fiist pair the shortest. 



Rudimentary pleopods appear on the abdominal 

 segments in tiiis stage. They are uniramous. 



The spination of the abdomen and the appear- 

 ance of tlie uropods is the same as in the preceding 

 stage. 



The notch at the distal end of the telson is shal- 

 lower tlian before. The tip of the notch now 

 reaches only the level of the most external pair 

 of the 7 pairs of terminal spines. The most ex- 

 ternal 2 pairs of terminal spines are farther apart 

 than in the first mysis. 



THIRD MYSIS 



Individuals of the third mysis stage (fig. 17) 

 measured from 3.7 to 4.4 mm., based on the ex- 

 amination of 6 specimens taken from the plankton. 



The distinguishing characteristics of the third 

 mysis stage are the appearance of a dorsal spine 

 on the rostrum and of well-developed pleopods. 



The first and second antennae are much the 

 same as in the second mysis. However, the exter- 

 nal branch from the distal segment of the first 

 antenna is now divided into 2 segments. The 

 same is time of the endopod of the second antenna. 



The first and second maxillae are essentially the 

 same as in the preceding stage except that the 

 scaphognathite of the latter has continued to in- 

 crease in size and bears 16 setae. 



The endopod of the second maxilliped has 5 seg- 

 ments instead of 4 and the exopod is 3-segmented. 



The first 3 paii-s of pereiopods are made up of a 

 protopod of 2 segments, an endopod of 4 segments, 

 and an exopod of 2 segments. The endopods of 

 the fourth and fifth pereiopods have 3 or 4 seg- 

 ments, the protopod and exopod 2 each. 



The pleopods are well developed although still 

 functionless at this stage. They are divided into 

 2 segments. 



The telson hiis become somewhat more elongate 

 with tlie result that the external pair of terminal 

 spines of the second mysis is now in a lateral 



position. The distal notch has become narrower 

 and shallower, and the height of the tip of the 

 notch is now at the level of the most external of 

 the 6 pairs of tenuinal spines. 



FIRST POSTLARVA 



Thirteen specimens of the first postlarval stage 

 measured from 3.8 to 4.8 mm. in body length. 

 All specimens of postlarvae were taken from the 

 plankton. 



No great metamorphosis takes place at the molt 

 which gives rise to the first postlarval stage (fig. 

 18). The pereiopods lose their exopods and the 

 exopods of the maxillipeds are lost or modified. 

 The pleopods take over the swimming function. 



The rostrum continues to bear 1 spine near its 

 base and is about equal in length to the eye when 

 the latter is extended forward. The spine at the 

 anteroventral corner of the carapace has disap- 

 peared. In most of the specimens examined, the 

 supraorbital spines were absent and a supraorbital 

 crest was developed. In a few cases, however, a 

 small spine remained. 



The inner and outer branches from the distal 

 segment of the fii-st antenna now have 2 and S 

 segments, respectively. The statocyst at the base 

 of the first antenna is fully developed. 



The endopod of the first maxilla is much re- 

 duced, unsegmented, and without setae. There is 

 no line of demarcation between it and the pedun- 

 cle. The same can be s?'d for the endopod of the 

 second maxilla, except that it still has 3 segments. 

 The scaphognathite, which now bears 18 setae, is 

 the dominant structure of the appendage. 



Striking changes have taken place in the appear- 

 ance of the first maxilliped. The endopod is un- 

 segmented, and the exopod has lost its setae. The 

 peduncle has become greatly widened and bears 

 numerous setae along its inner mai-gin. 



The second maxilliped has lost its exopod. The 

 endopod consists of 5 segments of which the distal 

 3 curve inward. The peduncle consists of 2 

 segments. 



The third maxilliped has undergone little 

 change, with the exception of the loss of the exo- 

 pod. The endopod still has 5 segments and the 

 I>eduncle 2. 



