Vimal Organs of Insects and Crustacea, 233 



the exterior end of these transparent bodies is always 

 rounded ; in which latter respect, and in the circumstance of 

 these ends not being covered with pigment, an approxima- 

 tion is seen to what is observed in the simple eyes. In some 

 other points of view the pyriform bodies in the eyes of the 

 Daphniae and Gammarus Pulex resemble more the lenses 

 of the simple eyes, as they are found aggregated together 

 in the Onisci, &c. These latter, however, (the aggregated 

 simple eyes) always possess, besides the spherical lens, a con- 

 vex projection of the cornea, and a round vitreous humour, 

 but never these transparent conical bodies. The eyes of 

 the Monoculus apus differ most from the aggregated simple 

 eyes; for in them the conical bodies are long, slender, and 

 very numerous. It is certainly best at present to separate 

 the compound eyes without facets of the lower crustaceous 

 animals, on account of their peculiarities, both from the com- 

 pound eyes with facets of insects and the Crustacea, and froni 

 the aggregates of simple eyes of the Onisci, &c. 



In the Daphniae and Branchipus the eyes are movable, 

 like those placed on pedicles in the Crustacea; in the Mon- 

 oculus apus, Gammarus PWex, and Cyamus Ceti, they are 

 immovable. " 



I shall conclude this descriptive part with a general survey 

 of the four chief forms exhibited by the eyes of the articu- 

 lated animals. 



1. Simple Eyes provided mth a Lens. — They contain a 

 lens and a cup-shaped expansion of the retina, the latter 

 being covered externally with a layer of pigment. The lens 

 is round, elliptical, or cylindrical, according to the form of 

 the eye. 



i. A lens and a vitreous body, with a circular band of 

 pigment between the two. Examples of this structure are 

 found in the scorpions and spiders; and, among insects, in 

 the simple eyes of the larva of the Dytiscus marginalis ; pro- 

 bably also in the Mantis religiosa and Gryllus hierogly- 

 phicus. 



ii. A lens without any evident vitreous body. Of this kind 

 are the simple eyes of the Tfettigonia tympanum, the Libel- 

 lula quadrimaculata, and a few others, together with those of 

 the *Scolopendras. 



2. Aggregated Simple Eyes provided mth Lenses, — The 

 simple eyes with lenses are commonly aggregated together 

 into a mass containing from twenty to forty. Each eye cor- 

 responds to a special convexity of the Cornea, but there are 

 no true facets. The genera Zulus, Lepisma, Cymothoa, 

 Oniscus, &c., afford examples. 



