THE COPEPOD GENUS PARANTHESSIUS — ILLG 411 



It will be noted that, while the full armature of the adult limb is not 

 present in this larval state, there are included in the complement of 

 the terminal segment spines or setae which finally appear, in the adult 

 stadium, on the second segment. 



The urosome is 4-segmented, composed of the sixth and seventh 

 thoracic segments and two abdominal somites. Of these latter, the 

 terminal segment is the longer. The fifth legs are short, the length 

 equalling about one and one-half times the width. Of the terminal 

 setae, the lateral is much the longer. The sixth leg is represented by a 

 single seta. The single pair of spinule rows of the seventh thoracic 

 segment comprise about twenty spinules on each side (fig. 34, L). 



The caudal rami are relatively short and wide ; the ratio of length 

 to width is about 2.6. Total body length, exclusive of setae of caudal 

 rami, is about 0.9 mm. No characters for differentiation of the sexes 

 were found. 



Remarks. — The single specimen upon which this species was founded 

 was taken swimming in the plankton in the Puget Sound region. It 

 was diagnosed as a male in the original description, with, however, one 

 lapse where it was referred to as female in the legend for the illustra- 

 tions (p. 90). The character that convinced the describer of the 

 generic distinctness of the species was the 2-segmented rami of the 

 swimming legs. This feature, together with the spine shown on the 

 genital segment and the general configuration of the rostrum in the 

 figures of the type, I find ample evidence to identify Thompson's 

 specimen with the fourth copepodid developmental stage, as borne 

 out by the series of specimens available to me which I consider to 

 represent the same species. Additional information from my series 

 serves to explain some other apparently anomalous features of 

 Thompson's description. 



The following are the characters in which Thompson's description 

 and figures agree with my specimens : 



The rostrum and antennules are alike in these two instances and are 

 unique for the genus Paranthessius. The antennae, mandibles, maxil- 

 lules, and the terminal portions of the maxillae correspond reasonably 

 well. The maxilliped described by Thompson and illustrated as his 

 figure 8 resembles no such structure Imown among the lichomolgids. 

 However, reference to the mouth parts figured here as figure 34, K, may 

 serve to explain this appendage. The base of the maxilla, with its 

 very obvious muscle band, if the terminal portion were torn away and 

 the line of this disruption misinterpreted as part of the natural boun- 

 dary, might very well be delineated in exactly the appearance of 

 Thompson's figure. Also, the maxillipeds are so inconspicuous and 

 so overshadowed by these enlarged maxillae that it would be very easy 

 t o overlook them in a single dissection. 



