5¢4 THE SENSES AND 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, 
which 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 pineal 
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 
tothe 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. PANKRATH, O., ‘Das Auge der Raupen und Phryganidenlarven.’ 
Zeitsch. f. w. Zool. Bd. xlix., 1890. 
249. WatTASE, S., ‘On the Morphology of the Compound Eye of Arthro- 
pods.’ Studies from Biol. Lab. Johns Hopkins Univer., vol. iv., p. 287, 
1890. 
250. PARKER, G. H., ‘The Histology and Development of the Eye in the 
Lobster.’ Bull. Mus. Comp. Zool. Harvard Coll., vol. xx., 1890. 
251. PARKER, G. H., ‘The Compound Eyes in Crustaceans.’ Bull. Mus. 
Comp. Zool. Harvard Coll. vol. xxi., 1891. (This paper contains 
a valuable and very extensive bibliography). 
252. EXNER, S., ‘Die Physiologie der Facettirten-auge von Krebsen und 
Insecten.’ 8vo., Leipzig und Wien, 1891. 
