no. 3602 CRAYFISHES — HOBBS, HOLT, AND WALTON 13 



area that do not carry at least a few worms. Brown (1961, p. 7, 

 table iv) collected 100 crayfish from Sinking Creek and recovered 

 from them an average of 106 worms of all species per crayfish. This 

 is, by far, too high an average for all the crayfish in the stream since 

 Brown chose only large and not recently molted crayfish for his 

 studies. 



In this study, we present under "Specimens Examined" the number 

 of animals actually studied, selected in most cases from much larger 

 numbers. The collecting methods we used were not such as to allow 

 us readily to determine the number of worms per crayfish, but several 

 times one of us (Holt) has counted over 200 worms from a single 

 crayfish from other areas. 



Little is known as to seasonal fluctuations in branchiobdellid 

 populations. Young (1966, p. 576) has found that branchiobdellid 

 populations decline in the winter, which is in agreement with our 

 experience that, when crayfish can be taken, branchiobdellids and 

 their cocoons are almost always present. Some exceptions to this 

 are known: in streams polluted with waste from coal mines, branchi- 

 obdellids disappear before crayfish do; although crayfish (C. b. 

 bartonii) are present in the upper reaches of Little Stony Creek above 

 the Cascades, branchiobdellids are often rare, perhaps because of the 

 effect of severe winter temperatures or of lowered pH values in the 

 boggy headwaters of the stream; and branchiobdellids are usually 

 absent from recently molted crayfish and almost invariably so from 

 very small animals. The scanty available evidence, then, is not 

 adequate to form a basis for statements about fluctuations of branchi- 

 obdellid populations. 



Little is known likewise about methods of infestation, and what is 

 known is due mostly to the efforts of McManus (1960, pp. 422-424). 

 He found that the worms apparently do not move to the host when 

 the old exoskeleton is shed in molting and that the young crayfish 

 are not infested with worms for some time after leaving their mother, 

 conclusions supported by our observations and those of Young (1966, 

 pp. 573-574). Apparently crayfish obtain worms when they come 

 in contact with another animal bearing them. It must be noted, 

 however, that these conclusions are not in accord with ours concerning 

 the infestation of crayfishes by ostracods. Especially striking is the 

 presence of ostracods and the absence of branchiobdellids on freshly 

 molted hosts. 



As is true of the ostracods, the epizoic branchiobdellids carry in 

 turn their burden of epizoites: peritrichous protozoans. In addition, 

 some of the larger branchiobdellids of the area are parasitized by an 

 unidentified organism, which is found usually as a spherical mass 

 embedded in the worm's body wall tissues. Protozoologists who 



