392 PROCEEDINGS OF THE NATIONAL MUSEUM vol.99 



includes many degenerate parasites. The actual affinities of many of 

 these may very well prove to be with several of the main stocks of the 

 copepods when more emphasis is placed upon the details of structure 

 rather than upon the fortuities of ecological distribution. 



The copepods here considered are cyclopoids that show a tendency to 

 reduction of mouth appendages; this character defines the section 

 Poecilostoma. In these copepods the traits unifying the group are the 

 very features that provide the most vexing of technical difficulties to 

 the investigator. It is essential for any coherent presentation of the 

 relations of the poecilostomes to establish the details of structure of 

 the mandibles, maxillules, maxillae, and maxillipeds. These appen- 

 dages are simplified in construction, reduced in size to minute pro- 

 portions, closely appressed, and wedged into a very small area with 

 closely impinged surrounding structures, such as a labrum which en- 

 velops a great part of the oral space. Dissection is the only satisfactory 

 means of elucidating these features, because the normal arrangement 

 of the mouth parts is such that most are held somewhat perpendicular 

 to the axis of the body and even the best toto views often lead only to 

 confusion of the details. 



The present collection comprises symbionts of pelecypod mollusks. 

 They were obtained, however, as a portion of a series of poecilostomes 

 collected in association with a diverse array of invertebrates from the 

 Pacific coast of North America. The forms are all assigned to the 

 genus Paranthessius Claus, according to the definition to be established 

 here. Noteworthy is the fact that congeners of these commensals of 

 mollusks have been described as living in association with coelenter- 

 ates, holothurians, and ascidians, evidencing the tendency in the 

 poecilostomes to diversity of host organisms. 



The genus Paranthessius is perhaps one of the most generalized of 

 all the diverse copepod types customarily assigned to the family 

 Lichomolgidae. In the structure of the mouth parts there is found 

 as simplified and basic a plan as is encountered elsewhere in the family. 

 The arrangement of the SMimming legs of some of the representatives 

 is possibly the least specialized among lichomolgids. I refer here to 

 species in which the armature of the fourth endopodite retains on the 

 terminal segment five outgrowths (setae or spines). A prominent 

 character of most of the species of the genus is the tendency to sup- 

 pression of the ornamentation of the fourth endopodite. In the 

 extreme condition of this modification, existing in several Paranthes- 

 sius species, the armature of the terminal segment of the fourth endo- 

 pod is reduced to two outgrowths. 



The subdivision of the Lichomolgidae proposed by Gurney (1927) 

 into Sabelliphilinae and Lichomolginae would place Paranthessius in 

 the former subfamily. This separation does not, I feel, follow a 



