322 PHILOSOPHICAL TRANSACTIONS. [ANNO J 782. 



our houses, instead of flower-pots, dishes containing some conferva rivularis, a 

 plant to be met with almost every where, shooting forth with the utmost luxuri- 

 ancy in all water basins, in all tubs and vessels in which water is kept. Is it 

 possible, after all this, he says, not to believe, that the Creator has multiplied 

 this vegetable with a similar view to our benefit ? This benefit we may now, with 

 some confidence, apply to our preservation, by honouring this vegetable with a 

 place in those of our own rooms which are exposed to the sun, and keeping it 

 alive as long as we please ; which may be done by only pouring every day fresh 

 water on it, and squeezing gently now and then out of it the dephlogisticated air 

 with whicli the whole mass swells up almost as soon as the sun casts its 

 rays on it. The water itself, in which it has been immersed, will now perhaps be 

 considered as too precious to be thrown away, as useless and deprived of that very 

 principal of animal life, of which he had demonstrated it to be highly pregnant. 



XXVlll. A Microscopic Description of the Eyes of the Monoculus Polyphemus 

 Linnai.* By Mr. William Andre, Surgeon, p. 440. 



The wonderful structure of the eyes of insects in general, most commonly 

 illustrated by that of the libellula, or dragon-fly, cannot fail of striking with 

 astonishment the naturalist who investigates the works of the great Creator in 

 his most minute productions. According to Leuwenhoeck, Hook, and others, 

 the cornea? of most insects are made up of an infinite number of small, trans- 

 parent, horny lenses, each resembling, in some degree, a small magnifying glass. 

 This structure prevails in the corneae of insects in general ; but the monoculus 

 polyphemus, or king crab, is, among others, an exception to this rule. 



The monoculus polyphemus, or king crab, is a crustaceous animal found in 

 all the seas surrounding the continent of America and the West-India islands, 

 and which frequently grows to a very large size. We shall describe so much of 

 the monoculus only as is necessary to point out the situation of the eyes, which 

 have been considered as 2 in number only, though in reality they are 4. The 

 largest piece of the crustaceous covering of this animal, when separated from 

 the rest of the shell, has very much the shape of a barber's basin, or the fore- 

 part of a woman's bonnet. The eyes are a part of the shell, or as Linneus ex- 

 presses it, they are testae innati.-J- They may be distinguished by the terms 

 large and small, or lateral and anterior. If the shell were divided fairly in half, 

 the large eyes would be nearly in the centre of each piece, and the small ones on 



* In a preceding part of the Philos. Trans., viz. Vol. 20, N° 2+o", p. 394, is an interesting 

 observation relative to the eyes of this animal, by Mr. Petiver. See vol. 4, p. 325, of these 

 abridgments. 



+ This being the case, the eyes can enjoy no motion ; in which particular, as well as in some 

 others, the monoculus polyphemus differs from the genus of crabs, whose eyes are placed on petioles, 

 or stalks, and are moveable.— Orig. 



