On two American Species of the Genus Hydrachna. 355 



Beneath the membrane in front, are situated two distant, dark brown 

 eyes, visible only from above. By a close microscopic examination, 

 these eyes are observed to be composed of two concurrent eye- 

 lets,* of which the anterior is the larger. They are capable of a 

 slight and quite peculiar motion to and from the shell. 



Beyond the anterior margin of the body, between the palpi, 

 extends a beaked projection which is connected with the organs of 

 manducation beneath. These organs are situated on a broad and 

 somewhat triangularly shaped pedicel which is capable of a slight 

 lateral motion independent of the body. From two sockets on each 

 side on its front margin arise the palpi (Fig. 3.), between which are 

 observed two short rounded projections (e, e,) which have been 

 considered a bifid linguette. Immediately below projects a rostri- 

 form mouth, (/"). 



The jpalpi are quinqui-articulate and usually arcuately inflexed, 

 the terminal joints being bent downwards. Their general form is 

 obclavate. The hasal joint is small, short and cylindrical. The 

 second is laterally compressed. In a perpendicular view, it appears 

 to increase with a curve from its base to the apex, where the anten- 

 nae, as seen in this view, attain their maximum diameter. When 

 amputated and placed on its side, (Fig. 6.) it exhibits a breadth 

 about equal to its length and more than double that of the succeed- 

 ing joint. Two or three setse are observed on both the interior and 

 exterior sides. The third is a short subcylindrical joint, provided 

 with a long seta near its outer base and another on its inner apex. 

 ^he fourth is more than double the preceding in length. On its in- 

 ferior surface arise three or four setae. The ffthf is a movable 

 corneous digit, bearing at its apex two minute hooks, one of which 

 only can be seen in a vertical view, and this with difficulty, unless 

 the antennae are separated from the body. 



The rostriform mouth is inserted nearly at right angles with the 

 body, and is curved forward, (Fig. 4.) This rostrum is composed 

 of two laminae, which form a sheath to an exsertile ligula. The 

 laminae when protruded separate and exhibit a lancet-like form, 

 (Fig. 5.) 



* A reexamination of the species heretofore described, would probably prove 

 that the eyes are in ail instances double ; the eyelets being separate Avhen there are 

 four eyes, and concurrent when apparently but two. 



t This has been considered an appendage to the preceding; as it is itself how- 

 ever terminated with appendages, it is here recognized as an independent joint. 



