TYPES OF VERTEBRATE AND INVERTEBRATE EYES 21 



Chap. 12, p. 149). Lastly, the " crystal cones " belonging to each com- 

 ponent of the composite faceted eyes of certain Crustacea are partly 

 " cellular," partly " vitreous," and will be described in the section dealing 

 with the eyes of arthropods (Figs. 81, 95, Chap. 11, pp. 119, 133). 



Single and Composite Eyes. — Groups of single eyes or ocelli, each 

 with a separate lens, may be aggregated in a single organ which is best 

 termed a composite faceted eye. These composite faceted eyes have to 

 be distinguished from eyes having a compound retina, as in the lateral 

 eyes of vertebrates, in which only one lens is present, but there are three 

 layers of sensory cells : rod- and cone-cells, bipolar-cells, and ganglion- 

 cells. Unfortunately the term " compound " has been used by different 

 writers to denote both the aggregate or composite faceted eyes and those 

 in which the retina is made up of two or more layers of sensory cells. 

 However, in many insects and Crustacea the two conditions are combined, 

 the eye being both composite and compound. Thus the confusion in 

 terminology is not so great as it might have been if they were separate. 



The Relation of Ocelli to Composite Faceted Eyes. — A consider- 

 able amount of interest has been taken in the relation which the simple 

 eye having a single lens has to the composite 

 lateral eyes which are typical of many adult 

 insects and Crustacea. The two types of eye 

 may be present together in a single adult 

 individual, or ocelli may occur in the larva 

 which are replaced or supplemented by com- 

 posite faceted eyes in the adult. The ocelli 

 may be lateral or median or may be both 

 lateral and median in position. They may 

 occur singly or in groups (Fig. 16). Fusion of 

 a pair of median eyes to form a single unpaired 

 median eye is common, or in some instances 

 where two pairs of median eyes are close 

 together the members of one pair, usually the 



anterior, will join, while the posterior pair of ocelli remain separate ; 

 a group of three simple eyes thus arise from two pairs of such eyes, 

 which originally were situated one behind the other, and between the 

 lateral faceted eyes (Fig. 72, Chap. 11, p. no ; Fig. 249, Chap. 24, p. 361). 

 The fused eyes frequently show regressive characters, being small and 

 imperfectly developed as regards structure. Ocelli, moreover, are often 

 present in the larval stage which disappear completely in the adult 

 (Fig. 78, Chap, n, p. 117). An interesting departure from the general rule 

 that the median eyes are simpler in type and less developed than the 

 lateral eyes is found in the scorpion, in which the lateral eyes (Fig. 94, 



Fig. 16. — Solitary and 

 Aggregated Eyes of 

 an Annelid — FLemopis 

 sanguisuga. 



(After Ariens Kappers.) 



