SIGHT IN CRUSTACEA. 479 



greater number of the larvse of perfect insects, simple eyes alone exist ; thus 

 the larvae of the bee have two simple eyes ; in the larvae of the Dytiscus beetle, 

 there are six on each side of the head. Some larvae are destitute of eyes, as 

 those of Hymenoptera. 



The eyes of the Myriapoda, like those of all the higher Annulosa, resemble 

 the eyes of insects ; they consist, however, for the most part, of conglomerate, 

 i. e., of simple eyes grouped together ; compound eyes are more rarely met with, 

 and when they occur, are of large size. The common Millipede ( Julus terres- 

 tris) has 28 eyes, placed in 7 rows, which form almost an equilateral triangle. 

 Many Myriapoda are blind. 



The Arachnida have only simple eyes or stemmata, but these are of very 

 perfect structure ; the pupillary aperture of the choroid is sometimes provided 

 with muscular fibres, which enable it to be contracted. The number of these 

 simple ocelli varies, being only two in the Chelifer and many mites, 4 in the 

 Phalangia and other mites, 6 or 8 in the Araneida, and 10 in the Scorpion. 

 When present in large numbers, they are often of different size in the same 

 animal, and are either closely crowded together on the top of the head, or 

 placed laterally on the cephalo-thorax, or even on the middle of the upper 

 surface of the abdomen. The eyes of spiders are often very bright, and of a 

 sapphire color ; the peculiar glare of the eyes of some spiders and scorpions, 

 depends on the difference in color between the circumference and centre of the 

 eye, the former part being pale, and the latter dark. The eye of the Tarantula 

 has a bright red centre, and an amber-colored margin. Many of the lower 

 Arachnida have no eyes. 



The Crustacea possess both simple and compound eyes. The simple eyes 

 are never more than two or three in number ; in some of the smaller Crustacea, 

 however, as in Daphnia, a number of these are grouped together, forming a 

 conglomerate eye, and are covered by a common cornea ; they are, moreover, 

 movable. The compound eyes resemble those of insects ; but they are placed 

 at the extremity of movable peduncles, at least in the highest Orders, so that 

 they can be turned in any direction. The corneal facets are usually hexagonal ; 

 sometimes, however, they are square, as in the lobster and shrimp. Among 

 some parasitic or fixed Crustacea, as in the Cirrhopods and others, eyes are 

 entirely absent, but, even in these species, they are present in the larval stage. 

 The eyes in the Annelides are still more simple ; they consist merely of an 

 expansion of the optic nerve, covered by a transparent membrane, formed by 

 the cuticle, and having behind it a layer of black pigment, sometimes perfo- 

 rated in its centre. In some species, a small transparent, spheroidal, refractive, 

 body or lens is met with, as in Alciope, the Leech, the Nereids, and others ; 

 but a lens is not always present. The leech, Hirudo medicinalis, has ten eyes, 

 which appear as small dark elevated dots, arranged in a semicircle on the fore 

 part of the head ; they have no pupil. The Eunice has four eye-spots on the 

 posterior part of the -body ; in the Nereis, there are four arranged in a quad- 

 rangle on the surface of the head. In a curious worm, named the Polyoph- 

 thalmus, besides a group of ocelli upon the head, there is found a succession 

 of other smaller ones, arranged in pairs, one on each side of every segment of 

 the long body ; all of these eyes have a minute refractive body or lens. In 

 cases where such a lens is wanting, the animals must be restricted to the mere 

 capability of distinguishing light from darkness, and color without form. Even 

 where the lens is present, it is doubtful whether any distinct image is formed, 

 except, perhaps, for very near objects. 



In the Annuloida, eye-spots are seen in the worm-like Scolecida, in the Ro- 

 tifera, and in a few of the Echinodermata. In the first group, may be men- 

 tioned the Planaria and the larval stage of Distoma and Monostoma, but the 

 Entozoa generally, are destitute of pigment spots. Amongst the Rotifera, 

 most species have two ocelli, but in some they are combined into a single spot, 

 resting on the centre ganglion of the nervous system, at the fore part of the 

 body. In the Echinodermata, eye-spots are seen at the ends of the rays of a 

 few star-fishes, and around the lower opening of the alimentary canal in some 

 Echinida. 



In a few of the Coelenterata, pigment spots are present, either destitute of 

 a lens, variously colored, and placed on the central nervous ganglion, as in 



