134 PHILOSOPHICAL TRANSACTIONS. [aNNO 1771. 



the shell is covered, and to which the dirt sticks, is only visible with the mi- 

 croscope. Mr. M. examined several of these, at different ages, and at different 

 times of the year, and found them all rough ; whereas every one of those of the 

 new species is smooth. He calls this new species the white smooth bivalve, to 

 distinguish it from another, the shell of which is white and rough ; and from 

 that of the above-mentioned authors, which he calls the sordid, in allusion to 

 the dirty shell in which it is often found. 



The smooth white bivalve has 5 capillary threads at each antenna, 4 of which 

 are at top, and the 5th somewhat lower. The sordid appears to have 10 at each 

 antenna; in several, the antennae appear yellowish, and their basis seems to 

 consist of 4 rings. It is by means of these antennae, which are real fins, that 

 the animal changes its position, from one place to another, being able to move 

 them several ways; when it has a mind to move fast, they are first extended 

 straightways, and appear like two bristles ; in an instant the threads are un- 

 folded, and the animal swims with great quickness. As for walking, it some>- 

 times joins the threads, sometimes unfolds only a single one, and sometimes 

 scatters them about all together; sometimes it bends them between the valves, 

 which are opened towards the place of the eye; it often hides one or both of 

 them under the breast between the 4 legs; these antennae seem to afford as great 

 an amusement to the animal, as they do to the spectators. At the place where 

 the head joins the body, towards the border of the hinge of the shell, is per- 

 ceived a little black spot, which is the animal's eye. This extraordinary situation 

 of the organ of sight on the neck seems astonishing ; every thing that is new 

 is so, but the surprize arises only from the narrowness of our ideas. 



Some aquatic insects have the eye in the forehead, others at the bottom, on 

 the fore or back part of the head, at the side or under it ; nay there are some 

 whose head consists of the eye only. The breast jets out a good deal towards the 

 opening of the shell, and constitutes the greater part of the animal's body. The 

 feet, mouth, and little bristles are placed on it. 



There are 4 feet, whose position resembles a good deal that of quadrupeds, 

 only that their reciprocal bent is more marked. The two foremost are at the 

 top of the breast, in the part where it appears most sloped. They are white, 

 transparent, and jointed, bent towards the back legs, and terminated by two 

 points in the shape of claws. The joints have very thin hair on the inferior 

 part. The 2 hind legs are fixed to the lower part of the breast. They are 

 longer than the fore legs. Each joint has a couple of small threads at the end, 

 and each leg terminates in a claw somewhat lengthened; as to the rest, they are 

 like the fore legs, and bend towards them. The bivalve insect makes use of its 

 claws, not only to walk on the conferva, some parts of which are true labyrinths. 



