On two American Species of the Genus Hydrachna. 357 



The interior organs fill but partially the coriaceous covering of the 

 body, and the usual appearance presented, is that of a small abdo- 

 men within an enveloping shell. From the extremity of the inner 

 abdomen a duct is visible passing to the anus ; others proceed from 

 the same point to the exterior margin of the plate, (Fig. 1, b.) de- 

 scribed as terminating the body where there are small holes (Fig. 2, 

 c, c.) which are the stigmata. The faeces of these animals are white. 



The space between the coriaceous covering of the venter and the 

 internal organs above described, is occupied in the gravid female, 

 either in part or entirely, with ovary sacs. The largest have an 

 oval form, an amber yellow color, and a length equal to about one 

 fourth the breadth of the abdomen. The smallest are white and 

 spherical. Between the two extremes of size, there are correspond- 

 ing variations in the shades of color. When extracted from the 

 body they appear slightly translucent. 



The young animal shortly after birth has somewhat the appear- 

 ance of a transparent globe with a brownish nucleus. It has only 

 the three anterior pairs of legs. The palpi project at one extremity 

 in the form of a beak, and a short distance back on each side, there 

 are two concurrent eyelets. The animal in this state scarcely moves 

 itself, when taken from the branchiae of the clam in which it lies 

 partially imbedded. When more developed its back is of a dirty 

 brown color, the central white line not being apparent. 



The following are the two species from which the above descrip- 

 tion has been derived. 



Hydrachna formosa. 



Body oval, in length twice its breadth ; slightly the broadest pos- 

 teriorly ; irregularly punctate ; frontal parts and lateral edges yel- 

 lowish, translucent ; marked on its back with a forked line some- 

 what resembling the letter Y ; extremity of the abdomen w r hite 

 above ; dark internal vessels varying from a light to a dark chesnut- 

 brown. Palpi and legs diaphanous, light green. When the palpi 

 are flexed in their natural position, a spine proceeds apparently 

 from the extremity of the fourth joint exterior to the fifth ; this 

 spine is one of the four that project from the inferior surface of the 

 fourth joint. Legs covered with long setaceous hairs (the tarsal 

 joint of the three anterior pairs excepted) ; diameter of the tarsus, 

 nearly one half that of the basal joint, greatest at its apex. Claws 

 doubly hooked ; hooks equal. (Fig. 7.) 



