356 On two American Species of the Genus Hydrachna. 



4 



The legs have six joints, exclusive of the basal or coxa], which is 

 immovable, and are covered with long, transparent, setaceous hairs. 

 The fourth or posterior pair is the longest;, being one and one fourth 

 the length of the body ; the second and third about equal the 

 body in length, and the first is three fourths of the same. The first 

 joint of the six anterior legs is very short, (Fig. 2, d.) ; the fourth 

 and fifth in each are the longest. The immovable basal joint of the 

 fourth pair is expanded into a broad lateral breast plate, and is uni- 

 ted by a suture with the corresponding joint of the third pair. The 

 last joint or tarsus in each is bifid at its apex and terminated by two 

 retractile claws, articulated with the inferior extremity of the foot. 



The genitalia are situated at the inferior extremity of the body. 

 In the female there appear externally two conchoidal plates, (Fig. 

 2, 6, &,) with their intermediate edges, or those by which they unite, 

 elevated in the form of a ridre. ' The extension of this ridge forms 

 a short tail which is terminated by four setae — two on each side. 

 These laminse open laterally. In the male the body is terminated 

 by a broad oval plate, conforming nearly to the curvature of the 

 abdomen, and having an elevation near its center. 



The colored internal organs of the body are visible through the 

 transparent membrane covering the body ; but as these organs are 

 mostly opaque their true nature cannot with certainty be determined. 

 Without however applying names they may be described to be, 1, 

 a broad white or yellowish white, central abdominal vessel, (Fig. 



1, e.) extending from a point anterior to the middle of the body to 

 the apex of the abdomen, where there is a connection with the anus; 



2, a brow T n or brownish black vessel, anterior to the commencement 

 of the preceding, (Fig. 1,/.) extending nearly to the line between 

 the eyes, whence it is apparently continued colorless to the mouth ; 



3, two lateral abdominal vessels which appear to be united with the 

 preceding, and lie along side of and partially overlap the central 

 vessel or organ £rst described, (Fig. 1 , g, g.) The abdominal 

 vessels are also visible below. 



To this partial overlapping of the white central abdominal vessel 

 by the two lateral, is owing the appearance of a narrow, irregular 

 white line along the centre of the abdomen, more or less forked in 

 front. The lateral vessels are sometimes set extended as to conceal 

 the central one, and consequently the medial white line is then in- 

 terrupted. 



