THE COPEPOD GENUS PARANTHESSIUS — ILLG 413 



to leave the host for varying periods, and numerous species have been 

 described that have never been taken in association with another or- 

 ganism. Several records of Paranthessius rostratus are derived from 

 collections of plankton from waters above cockle beds where the mol- 

 lusks are infested by this species. In the case of Paranthessius colum- 

 hiae I have recovered a male individual of the fifth copepodid stadium 

 from washings of the red alga, which almost invariably occurs as dense 

 tufts on the heavy siphon plates of Schizothaerus nuttallii of Tomales 

 Bay. The amount of migration of the adults and the relative wander- 

 ings of the larval stages would be interesting details to supply in a 

 complete biological investigation of this copepod. Large scale surveys 

 of the mollusk fauna of a given locality should provide information of 

 great interest in regard to the limits of specific infestation of given 

 hosts. 



PARANTHESSIUS PANOPEAE, new species 



Figures 35, 36 



Specimens examined. — Fourteen adult females, nine adult males, 

 one female of the fifth copepodid stadium, from a single specimen of 

 Panope generosa Gould, collected in Tomales Bay, Marin County, 

 Calif., May 23, 1943, P. L. Illg, collector. 



Types have been assigned the following United States National 

 Museum catalog numbers : Holotypic female, 85343 ; allotypic male, 

 85344. 



Description. — Female : The body is elongated although not so ex- 

 tremely as in P. columhiae. The metasome consists of five somites. 

 The cephalothorax is the longest segment. The relative lengths of 

 the succeeding somites are in the order of their arrangement, as are 

 also their relative widths. The somite of the first swimming legs is 

 the widest somite of the metasome (fig. 35, A). 



The urosome is 5-segmented, consisting of the sixth thoracic somite, 

 the genital somite, and three free abdominal segments. The ovisacs 

 are narrow, closely appressed to the abdomen, and elongate, reach- 

 ing to the ends of the caudal rami. The eggs are small ; a single ovisac 

 contains a hundred or more eggs. 



The rostrum is a long, pointed beak, directed posteriorly. It is 

 movable, and the supporting structures on the head are associated with 

 the bases of the antennules and antennae. There are no lateral auxili- 

 ary projections (fig. 36, A). 



The antennule (fig. 36, A) is long and slender; it comprises seven 

 segments. The second segment is the longest. The setal armature is 

 as follows : First segment — four setae ; second segment — nine setae ; 

 third segment — four setae; fourth segment — three setae; fifth seg- 



