338 



FISHERY BULLETESr OF THE FISH AND WILDLIFE SERVICE 



There is a series of plumose setae along the mar- 

 gins of the appendage. All of the setae are con- 

 siderably shorter than in the preceding stage. A 

 large spine is present about midway along the 

 inner margin of the basal segment. 



The segments of the exopod of the second an- 

 tenna have fused to form a flattened scalelike 

 structure. A series of 11 setae is present along the 

 inner and terminal margins, with a twelfth pres- 

 ent on the outer margin. The endopod has de- 

 veloped into an unsegmented rodlike structure 

 about two-thirds the length of the exopod. It 

 bears approximately 4 smooth setae, 3 of which 

 are terminal. The protopod consists of 2 

 segments. 



The first and second maxillae are essentially 

 the same as in the preceding stage with the excep- 

 tion that the scaphognathite of the second maxilla 

 has become enlarged and now bears 10 setae. The 

 first and second maxillipeds likewise show no ap- 

 preciable change. 



The third maxilliped is now longer than the 

 first 2. It consists of a 2-segmented protopod, a 

 5-segmented endopod, and an unsegmented exopod. 

 The endopod bears at least 5 setae, 4 of which are 

 terminal. The exopod bears approximately 6 

 setae. 



The 5 pairs of pereiopods have developed con- 

 siderably, and are the chief natatory structures of 

 the larva at this stage. In swimming they are 

 assisted by the 3 pairs of maxillipeds. The first 3 

 pairs of pereiopods are chelate and show a slight 

 increase in size from the anterior to the posterior 

 part of the body. They are composed of a 2-seg- 

 mented protopod, an endopod of 2 segments (the 

 distal end of the endopod is the rudimentary 

 chela), and an unsegmented exopod. The chela 

 bears 2 or 3 short terminal setae and the endopod 

 6. The fourth and fifth pereiopods are composed 

 of a 2-segmented protopod, a very short unseg- 

 mented endopod, and an unsegmented exopod 

 which is 3 or 4 times the length of the endopod. 

 The endopod bears about 6 terminal setae. 



The fii*st and second abdominal segments have 

 lost their dorsal spines, and those of the third 

 and fourth segments are much reduced. The dorr 

 sal spines on the fifth and sixth abdominal seg- 

 ments are still fairly prominent. In addition, 

 the fifth segment bears a pair of posterolateral 



spines, and the sixth segment bears 2 pairs of pos- 

 terolateral and 1 pair of posteroventral spines and 

 a ventromedian spine on its posterior margin. 

 Buds of the pleopods can be seen on the first 5 

 abdominal segments. 



The uropod now has a protopod which bears 3 

 spines on its distal border. The exopod and en- 

 dopod are about equal in length, however, the for- 

 mer is much the wider. The exopod bears about 

 8 setae on its inner margin, as well as 2 or 3 short 

 setae between the subterminal spine on the outer 

 margin and its distal edge. The endopod bears 

 about 6 lateral and '3 terminal setae. 



The telson beai-s 7 pairs of terminal spines and 

 a pair of small lateral spines. The median notch 

 in the distal margin of the telson is quite well de- 

 veloped, the height of the tip of the notch falling 

 between the lateral and terminal spines. The 

 depth of the notch and the level of its tip in rela- 

 tion to the spines on the telson are important 

 characters in differentiating the three mysis stages. 



SECOND MYSIS 



Specimens of the second mysis stage (fig. 16) 

 measured from 3.3 to 3.9 mm., based on examina- 

 tion of 8 specimens found in the plankton. The 

 distinguishing characteristic of this stage is the de- 

 velopment of rudimentary pleopods. 



The carapace now extends back, completely cov- 

 ering the thorax. The appearance of the rostrum 

 and the spination of the carapace are the same 

 as in the preceding stage. 



The branches from the distal segment of the first 

 antenna are now almost equal in length. Numer- 

 ous setae are present at the junction of the seg- 

 ments of the appendage. A bulge, which is the 

 developing statocyst, appears in the basal segment. 



A subterminal spine api^eai-s on the outer mar- 

 gin of the exopod of the second antenna. The 

 latter bears 8 setae along its inner margin, 4 ter- 

 minal setae, and 3 setae between the distal tip and 

 the subterminal spine. The endopod remains un- 

 segmented and rodlike, and in this stage is ap- 

 proximately half the length of the exopod. The 

 distal segment of the protopod bears a median 

 spine. 



The scaphognathite of the first maxilla has dis- 

 appeared, while that of the second maxilla has 

 increased in size and now bears 14 setae. 



