TYPES OF VERTEBRATE AND INVERTEBRATE EYES 27 



and only the median pair is present (Fig. 19, B, C). These appearances 

 are suggestive of the median paired eyes having originated from the mar- 

 ginal eyes, two of the central members of which, having become shifted 

 backwards, are left behind the others on the dorsal aspect of the head, 



A 



DAPHN 



Fig. 21. — Three Specimens of Cladocera, belonging to the Sub-order Bran- 



CHIOPODA OF THE CLASS CRUSTACEA. THE LATERAL EYES ARE SESSILE AND 

 JOINED TOGETHER IN THE MEDIAN PLANE TO FORM A SINGLE ORGAN. THE 



Cladocera are greatly reduced in size (1 to 2 mm.) and Segmentation is 



VERY IMPERFECT. 



A — Daphnia. B — Polyphemus. C — Leptodora. 



The eyes of Daphnia and Polyphemus are, relative to the size of the body, very 



large. 



ant. 1 : antennule. d. gl. : digestive gland. 



ant. 2 : antenna. /. : swimming feet. 



br.p. : brood-pouch. ht. : heart. 



E : eye. md. : mandible. 



(A : after Claus. B and C : after Gerstaecker.) 



while the remaining marginal eyes are carried forward and laterally^ 

 following the general growth of this region of the head. Whether paired 

 eyes have arisen in this way or have been evolved independently, the 

 foundation of a paired system of eyes is present in these simple Planarian 



