256 HYDRACHNA. 



behind ; that the head and body are united in one, forming a single or- 

 ganization, of what other authors subdivide as three, that is, head, thorax, 

 and abdomen. Earlier distinctions were founded on the figure of the 

 body, the presence, number, and position of the eyes, and other judicious 

 characters, such as may be readily recognized and sufficiently adapted 

 for scientific arrangement. For these, some modern naturalists desire 

 the substitution of certain minutire, sometimes of very difficult detection 

 and equivocal determination ; which may be possibly alike decisive when 

 discovered, such as the form and position of the mandibles, the place of 

 the mouth, the intervals and proportions between the joints and the 

 limlas, and other mattei-s. But these must be sought by naturalists 

 habituated to similar investigations, nor do they contemplate the pre- 

 servation of the subject itself entire, in freedom, or with the knowledge 

 of its habits. 



Hydrachna^ dwell exclusively in fresh water ; they are found in 

 marshes, ponds, lakes, slow and tardy currents, where a wider and more 

 ample space admit their pursuing a deviating course throughout the 

 brighter portion of the finer days of summer. The observer who stations 

 himself at a certain point of the brink, towards the margin, is assured 

 of being visited by each successively in its course. Let him advance, he 

 may anticipate that in passing onwards he shall be visited by another : 

 next, by a third, a fourth, and by many in succession. All are guided 

 evidently by a common impulse, as having a particular object in view. 

 Thus, they expose themselves to capture ; for the naturalist, by merely 

 sinking a white saucer obliquely in that direction from whence they 

 visibly journey, and raising the opposite edge leisurely, intercepts their 

 nearei- approach in farther retreat, and secures them. 



Such may be a useful lessou. The habits of those wild animals, 

 most valuable to the hunter, are thus rendered the means of their capture. 

 Should Hydrachn£B be alarmed, they always descend, seeking safety in 

 deeper and deeper water, the wonted resource of aquatic animals to 

 escape ; some there are, however, who under similar circumstances alwavs 

 endeavour to rise upwards. 



Certain species, undoubtedly, prefer considerable depths ; a few may 



