THE COPEPOD GENUS PARANTHESSIUS — ILLG 409 



Fifth copepodid. Female : The rostrum is very conspicuously de- 

 veloped in the 3-pronged pattern of the adult male. The lateral 

 spines are equal to two-thirds of the length of the central beak. The 

 appendages of the head all are identical with the adult condition. The 

 thoracic appendages of the metasome are all identical with those of 

 the adult. The small maxilliped establishes the specimen as a female, 

 since the adult condition of the swimming legs is taken to be conclusive 

 indication that this stadium is the ultimate larval copepodid. 



In the urosome the characters depart from those of the adult. The 

 fifth leg, however, approaches closely to the adult condition, although 

 it is perhaps slightly less elongate. It is, in actual measurement, at 

 least twice the bulk of the fifth leg of the adult male. Further, the 

 terminal armature presents the stout medial spine, longer than the 

 lateral seta. In the male, both adult and of this stadium, the longer 

 member of these outgrowths is the lateral, and neither attains spinelike 

 dimensions. 



The urosome is 5-jointed, but it represents different segments from 

 those present in the adult urosome of the same number of somites. 

 The first segment is the sixth thoracic somite, bearing the fifth legs. 

 The second segment is the seventh thoracic somite ; it is thus established 

 by the presence at its posterior margin of a spinule row and one seta, 

 comparing incompletely but satisfactorily to the sixth leg armature 

 of the adult male. The adult male, and also that of the fifth copepodid 

 stadium, show two setae for this member. In this subadult female, the 

 spinule row is but a single set of about 20 spinules on each side as 

 compared to the pair of double rows of the corresponding male. 



Three abdominal somites complete the urosome. Of these, the ter- 

 minal one is the longest. This segment bears the characteristic pair 

 of rows of spinules, exhibiting but eleven spinules on either side in 

 the specimen examined. The proportion of length of segments is 

 significant as it indicates clearly the rearrangement of segmentation 

 of the urosome that takes place during the final molt. The result of 

 this molt is a urosome in which the terminal thoracic segment and 

 the first abdominal somite are fused into the very long genital seg- 

 ment. In this modification, all the ornamentation of the sixth leg 

 disappears (except perhaps a reduced seta or two very inconspicu- 

 ously placed at the openings of the oviducts) . In the adult, the most 

 proximal of the three abdominal segments is the longest, and this fact 

 indicates that the terminal segment of the fifth copepodid is sub- 

 divided in the final molt. 



The caudal rami of the fifth copepodid are stubbier than those of 

 the adult. The proportion of length to width is about 3.3. The 

 length of the body measures 1.4 mm. 



