CONFUSION IN CLASSIFICATION. 353 



classes of articulated animals. The errors which have been 

 committed in this respect ought at least to teach us caution. 



Thus, the red, oval, six-legged animalcules, whose mobile 

 heads are furnished with a proboscis shaped like an angular 

 beak, and whose two palpi are large and semi-transparent, 

 the singular animalcules which, in June or July, are hatched 

 in the spongy stems of certain aquatic vegetables, notably 

 the Potamogeton natans, have been described * as forming a 

 peculiar genus of Arachnidae, the genus Achlysia ; and this 

 genus, created by Audouin, was ranked along with the Leptus 

 and Ocypete in the family of Microphthirae. Yet nothing is 

 less exact. These Achlysia are simply the larvae of a kind of 

 Hydrachna or water-acarus. To be convinced of this, you have 

 but to watch their development. At first very small and 

 pear-shaped, these larvae, deprived, like all larvae, of the 

 reproductive organs, rapidly increase in size. At the end of 

 a few weeks you will see them adhering to a leaf of potamo- 

 geton ; they thrust their proboscis into the stem, and cling to 

 it with their palpi. Little by little, the legs, the proboscis, 

 and the palpi, are drawn back towards the body, abandoning 

 the skin which has hitherto formed for each of these organs a 

 kind of horny sheath. From the larva state, the animal passes 

 into that of the nymph. But this nymph continues to feed 

 and enlarge ; proboscis, legs, and palpi grow thinner and 

 harder; claws, ciliae, and hairs are developed; and, finally, 

 through a fissure in the skin emerges the perfect animal, red as 

 wine, with eight feet, and about two millimetres in length. 

 * " Annales des Sciences Naturelles," i. 18. 



