358 On tivo America?! Species of the Genus Hydrachna. 



Length of the male, including the palpi ^V— 3 V of an inch ; of 

 female ^.'g — ^\ x)f an inch. Found most abundant in the Anodonia 

 cataracta, in one specimen of which there were forty four individuals ; 

 also in the Unio purpurata. Collected in the course of April. The 

 females usually contained numerous ovary sacs. 

 ' To the unaided eye this animal has a considerable resemblance to 

 a minute Coleopterous insect, the longitudinal white line appearing 

 to mark the separation of the elytra. 



H. pyriformis. 



Body, broad ovate, sub-pyriform, sub-globose posteriorly, breadth 

 across the abdomen, nearly twice that in a line with the eyes ; medi- 

 al abdominal line, white, scarcely forked, but broader anteriorly ; 

 dark internal vessels black. Palpi diaphanous : when inflexed, ap- 

 parently terminated by three spines of which the corneous fifth joint 

 is the central one ; the lateral are two out of the four which pro- 

 ceed from the inferior surface of the fourth joint. Legs, diaphanous, 

 covered with setaceous hairs ; tarsus of each leg slightly hairy ; one 

 third the diameter of the basal joint, smallest one fourth the distance 

 from the apex. Claws simple, small. 



Length ---^ of an inch. Found in the Alasmodonta undulata, dur- 

 ing the month of April. Fig. 9, is an outline of the body. 



Many points which should be noticed in the description of the 

 species, are probably described in the preceding remarks. The rea- 

 son for this arrangement, has already been stated. 



These species, appear better fitted for their peculiar parasitic 

 habits, than for the life of animalcules hunters, which Miiller ascri- 

 bed to the species he examined. Provided with long slender legs 

 unfit for natation, they sink almost immediately, when brought to 

 the surface of the water, notwithstanding a rapid motion of these 

 organs, and are only able to make their way along the bottom of the 

 vessel that contains them. In every part of their structure, they 

 appear to be, destined to lead the life of prowling adventurers, roving 

 over the bottom of streams for prey, and entering the habitation of 

 any unfortunate clam that may be open to them. Lender the man- 

 tle which covers the body of the clam, they find a snug retreat, where 

 they may revel in all the luxuries of which their sensual appetites 

 are capable. The mouth being composed of a pair of lancet-like 

 blades, they are enabled to prepare a place for this minute and del- 

 icate organ, previous to inserting it. 



