412 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 99 



The first and fourth swimming legs are figured by Tliompson and, in 

 segmentation and arrangement of ornamentation, conform exactly to 

 my description above. 



The fifth feet present a discrepancy, Thompson's figure depicting a 

 longer medial terminal seta. Whether great pains were taken to 

 depict this feature exactly in an illustration (Thompson, fig. 1) obvi- 

 ously designed to present the body habit of the copepod rather than 

 details of its anatomy might be subject to question. 



Three abdominal segments are shown in Thompson's illustration 

 (fig. 1), with a row of spines depicted along the boundary of the 

 second and third segments. I suggest that the spinules present on the 

 undivided terminal segment in my specimens could be readily so misin- 

 terpreted in casual observation. The seta of the sixth leg corresponds 

 in the two instances. 



The length (1.98 mm.) given by Thompson is greatly at variance 

 with my specimens. He does not state whether the caudal setae are 

 included in the measurement, but these structures are much less than 

 half the body length in extent and could not account for the discrep- 

 ancy seen. It may be that Thompson studied a specimen of a species 

 different from my examples, but there are so many points by which I 

 can reconcile these with his description that I do not deem it advisable 

 to add a new name for representatives of this genus. 



The specimens I have examined, including ovigerous females, adult 

 males, and immature stages were taken as commensals or parasites upon 

 the gills of clams. I have specimens collected from the mudflats near 

 the mouth of Tomales Bay, Marin County, Calif., and additional ex- 

 amples from the similar situation nearby, Bodega Bay, Sonoma 

 County, Calif. Further information as to the extent of occurrence of 

 the copepod and the seasonal fluctuation of incidence and abundance 

 would be of considerable interest. The thorough work of MacGinitie 

 (1935) on the population of Elkhorn Slough, Monterey County, Calif., 

 did not discover the presence of Paranthessius columhi-ae^ although 

 the abundantly occurring principal host was sufficiently scrutinized to 

 bring forth numerous records of the encysted larvae of tapeworms. 

 Even so inconspicuous a copepod could not have eluded such search. 

 From observation of the living animals m situ upon the host, I can 

 attest the great transparency and nearly complete invisibility of this 

 copepod. Even under close scrutiny of the gills of the mollusk no more 

 than a minute dark thread of about a millimeter or tAvo in length be- 

 trays the parasite. This thread is the gut, which has a heavy wall. 

 Whether the dark color is pigmentation of the wall itself or a prop- 

 erty of the contents of the gut, I did not determine upon living cope- 

 pods, and in preserved specimens the coloration rapidly disappeared. 

 The occurrence of Thompson's specimen in the plankton is not diffi- 

 cult of explanation. It is a frequent phenomenon for lichomolgids 



