SEA-FISHERIES LABORATORY. 447 



meets the peduncle of the other side in the median line 

 above the first cephalic sternum. This proximal portion 

 is not seen from the exterior. (2) A shorter and broader 

 distal portion, which is the part seen from the exterior. 

 The two parts are separated by a region of soft chitin, 

 which allows of considerable movement between the two 

 parts. At the distal extremity of the peduncle is a small 

 black area — the cornea — which defines the external 

 surface of the eye. The cornea would be almost circular 

 were it not for the presence of two small tubercles w T hich 

 invade the dorsal border of the black area, and thus give 

 it a somewhat irregular shape. The cornea is composed 

 of a thin layer of chitin, which is continuous with 

 the thick, strongly calcified integument covering the 

 remainder of the distal part of the peduncle. 



The thick calcified integument has the usual layer of 

 epidermis beneath, resting upon a basement membrane. 

 In the region of the eye, the cellular layer on the inner 

 side of the cornea is extremely thick, and is known as the 

 ommateum. On the inner side of the ommateum is a well- 

 defined basement membrane, which is continuous with 

 that of the ordinary epidermis. The ommateum, 

 therefore, may be regarded as a differentiation of the 

 epidermis, with which it is continuous. 



An examination of the surface of the cornea reveals 

 the presence of numerous small hexagonal facets. Vertical 

 sections through the eye show that this sub-division of the 

 cornea is not merely superficial, but is continued through 

 to the inner side of the chitinous layer. The cornea may, 

 therefore, be said to be composed of numerous hexagonal 

 prisms, which are packed closely together. On the inner 

 side of the cornea the ommateum is divided into numerous 

 elements, which correspond in number and position to the 

 corneal facets. Each element of the ommateum is known 



