22 THE PINEAL ORGAN 



Chap, ii, p. 132) resemble the simple eyes or ocelli of insects (Fig. 9, p. 14) 

 while the pair of larger median eyes differ from them in having the retinal 

 cells arranged in groups, as in the composite eyes of certain insects and 

 Crustacea ; they resemble the simple eyes, however, in having a non- 

 faceted single lens (Fig. 95, Chap. 11, p. 133). The whole question of 

 the existence of the " median," " frontal," or " accessory " eyes in 

 larval and adult insects and Crustacea, and the frequent disappearance 

 of the larval eyes which takes place in the change to the adult form, is of 

 the greatest importance in understanding the nature of the median or 

 " pineal eyes " of vertebrates. We shall, therefore, attempt to give a 

 brief sketch of some of the more essential points of these and of the 

 lateral eyes in invertebrates, with the object of making a comparison 

 between the two and showing the differences as well as the similarities 

 in their structure and position. 



Beside the median and lateral eyes of the head, eyes of different type 

 appear in various parts of the body, e.g. on the margin of the umbrella 

 in coelenterates, on the edge of the mantle in certain molluscs, and on 

 the back in Chiton. The general form and connections also vary ; thus 

 they may be stalked, sessile, or imbedded in a socket lined by a serous 

 membrane. Some of the best examples of stalked eyes occur in the 

 Crustacea, e.g. the crayfish and the shrimp ; among the Mollusca — snails 

 and slugs ; and in vertebrates the " parietal-organ " or " pineal eye." 

 In the lateral stalked eyes of the invertebrates, mobility is dependent on 

 the movements of the stalk, which are carried out by muscle fibres within 

 the stalk. The small pineal eye — " parietal organ " — of living vertebrates 

 is usually immovably socketed in the parietal foramen, where it is sur- 

 rounded by dense fibrous tissue. We have no direct evidence of 

 its mobility in extinct animals ; the parietal foramen in many of the 

 extinct reptiles and amphibia is, however, much larger than in any living 

 species, and it is within the bounds of possibility that movements of the 

 stalk or stalks similar to those which occur in the stalked eyes of 

 invertebrates may have taken place. 



In discussing these different types of eye it will be unnecessary to 

 describe in detail certain varieties of eye, which although of considerable 

 general interest, yet have no special bearing on the subject with which we 

 are dealing. It is important, however, to note that a knowledge of the 

 structure of certain atypical eyes, which owing to their position cannot be 

 regarded as homologous with those of the head, although they have many 

 points in common with these, is nevertheless of considerable value. The 

 structural resemblances must have arisen in response to similar environ- 

 mental factors and similar somatic conditions, e.g. the action of light on 

 skin and pigment granules contained in the epithelium and subcutaneous 



