;c 4 THE SENSES A.\P SENSORY ORGANS. 



same species. Again, in the Lamellibranchs eyes both simple 

 and compound, and exhibiting very variable types of structure, 

 are scattered along the edges of the mantle, and some of these 

 present the most complex structure, even approximating the 

 vertebrate eye in complexity, as in Pecten. I propose to term 

 such eyes sporadic. 



In the Arthropoda and Vertebrata the eyes are cephalic, 

 but several types exist. The paired cephalic eyes of Vertebrates 

 and Mollusca, and the paired compound eyes of Arthropods, 

 \\hich differ greatly in structure, resemble each other in their 

 relation to the central nervous system ; but they are similar 

 only in position ; whether they are derived from a similarly 

 placed pair of eyes in some common ancestral form may be a 

 matter of speculation ; but no speculation can derive them 

 from sporadic eyes, the position of which is not only variable, 

 but is rarely cephalic. 



Yet a third type of eye is known, the remarkable median 

 unpaired cephalic eyes of Vertebrates, the so-called pinrnl 

 eyes, which in some respects resemble the ocelli of many 

 Insecta. Whether the paired cephalic eyes are derived from 

 unpaired median cephalic eyes is again a matter of speculation. 

 The eyes of Arachnids tend to the belief that they probably 

 have so arisen, and there are facts connected with the relation 

 between the ocelli and compound eyes of Insects which point 

 to the same conclusion. It will be useful, however, to designate 

 such eyes median cephalic eyes, since it is only by speculation 

 that the paired cephalic eyes can be compared with the median 

 cephalic eyes, which in many points are very different. 



248. I'AXKRATH, O., ' Das Au^e der Raupen urnl 1'hryganidenlarven.' 

 Zeitsch. f. \v. Xool. Bd. xlix., 1890. 



249. \Y.vi ASK. S., ' On the Morphology of the Compound Kye of Arthro- 



pods.' Studies from Uiol. Lab. Johns Hopkins Uimer.. vol. iv., p. 287, 

 1890. 



250. I'AKKI.K, G. H., 'The HUtoln^y and Development of the Kye in the 



Lobster.' Hull. Mus. Comp. Xnol. H Tv.ird Coll., vol. xx., 1890. 



251. I'AKKI.K, (',. H., 'The Compound Eyes in Crustaceans.' Bui 1 . Mus. 

 Comp. X.ool. H.irvard Coll., vol. xxi., 1891. (This paper contains 

 a v.-ilu .ihli- and very cxunsive bibliography). 



252. LXM K, S., ' Die 1'hvsiologie der Fa- .fit tcn-nu^e von Krehsen und 



Livcten.' 8vo , Leipzig und Wien, 1891. 



