no. :;602 CRAYFISHES HOBBS, HOLT, AND WALTON 7 



side of the thorax. A few were taken from the mouth parts." In 

 studying the incubation and hatching of the eggs of E. illinoisensis, 

 Stamper (1957, p. 50) reported that 90 percent of the crayfishes 

 examined carried eggs of the ostracod "attached to the hooked setae 

 and/or the coxal filaments. Both of these structures are located 

 underneath the carapace of the crayfish." Westervelt and Kozloff 

 (1959, p. 239) stated that E. neglecta (= U. neglecta) occurs principally 

 on the same areas of the crayfish as U. erichsoni and U. caudata. 



Two of us (Walton and Hobbs) have attempted to discover whether 

 or not several of the species in the Mountain Lake area are limited 

 to certain anatomical regions of their crayfish host. As might be 

 expected, they are not found on exposed surfaces except where there 

 are setae to which they may cling or grooves in which they may 

 obtain support. Particularly do they seem to congregate on the 

 gnathal appendages, where, on occasional crayfishes, scores of them 

 may be found moving about or clinging to the setae. Far fewer 

 individuals are found among the articular membranes and setae 

 associated with the ambulatory appendages, but certain species are 

 more abundant on the abdomen. Not one ostracod has been ob- 

 served in the branchial chambers of the crayfishes examined! There 

 seems to be no absolute specificity of microhabitat on the crayfish 

 for any of the ostracods observed although if two or more species 

 occur on the same crayfish, as they frequently do, one may be found 

 more abundantly in one area than elsewhere. We have found, for 

 example, that in the associations of Ascetocythere asceta and Dactylo- 

 cythere chalaza on Cambarus carolinus, the latter is more abundant 

 on the abdomen and As. asceta among the gnathal appendages. 



None of the ostracods in the Mountain Lake area occurs in all 

 parts of any of the drainage systems and, although they are obviously 

 subject to restrictions, no analytical data are available to demon- 

 strate the nature of these limiting factors. Two of the species, 

 As. asceta and Dt. chalaza are confined to the exoskeletons of the 

 burrowing crayfish, C. carolinus, which tunnels its way to ground 

 water. Whether it is the absence of competition with other os- 

 tracods, the groundwater habitat, an obligate relationship with 

 the crayfish, or a combination of, or none of, these factors that is 

 restricting these ostracods remains to be demonstrated. Altitudinal 

 correlations exist with at least two of the species — Donnaldsoncy there 

 scalis has not been found below approximately 670 m, and Ankylo- 

 cythere ancyla above 475 m; however, it is unlikely that altitude is 

 serving directly as a barrier to them. To our knowledge, not a 

 single limiting factor in the ecological distribution of any entocytherid 

 has been identified. 



If, as generally conceded at the present time, the ostracods of the 



