MUSEUM OF COMPARATIVE ZOOLOGY. 5 



anterior edge of the carapace. The form of the eye-stalk approaches 

 that of a short cylinder terminated by a hemisphere. The cylindrical 

 part of the stalk resembles the general surface of the body in that it is 

 covered with a tirm, calcified cuticula. Excepting a portion of the 

 surface next the rostrum, the whole of the hemispherical part during 

 life is black, and covered with a flexible cuticula. The black area de- 

 fines the position of the retina. That portion of the hemispherical 

 surface which is not black, and which faces the rostrum, is covered with 

 a peninsula-shaped piece of inflexible cuticula. A broad isthmus of 

 the same kind of cuticula connects this with the shell of the cylindrical 

 part. The absence of the retina from the peninsula-shaped portion of 

 the hemisphere is due in all probability to the fact that the field of 

 vision for this part of the hemisphere is cut off" by the rostrum. The 

 remainder of the hemisphere, that part on which the retina is devel- 

 oped, faces away from the lobster's body, and its field of vision is not 

 permanently obstructed by any part of the animal. 



A section perpendicular to the sm-face, and cutting the eye-stalk in a 

 region where the cylindrical and hemispherical parts unite, is shown in 

 Figure 26. The thick, calcified cuticula of the cylindrical part is indi- 

 cated at eta. On the inner surface of this cuticula is a thin hypoder- 

 mis (Ji d.). The hypodermis is bounded on its inner face by a basement 

 membrane (jnh.). The cuticula of the hemispherical part {cm.) is thin 

 and flexible. It can be designated by the name corneal cuticula. 

 (Compare Patten, '86, p. 544.) Eesting on the deep face of the corneal 

 cuticula is the thick cellular layer, named by Lankester and Bourne the 

 ommateum (omm'.). The proximal face of the ommateum is limited by 

 a basement membrane, which is continuous with tliat bounding the 

 corresponding face of the undifferentiated hypodermis. The onunateum 

 is continuous with the hypodermis, and in fact can be regarded as a 

 thickening of that layer. Can-iere ('85, p. 169) has already pointed 

 out in the eye of Astacus a similar relation between the hypodermis 

 and ommateum, and he believes that this relation holds good for all 

 Decapods. 



On inspecting the external face of the corneal cuticula, one finds it 

 divided into an immense number of square facets, one of wliich is shown 

 in Figure 2. Although as a rule the outline of the facet is squai^e, it is 

 not invariably so ; for on the margin of the retinal area close to where 

 the ommateum passes over into the undiflerentiated hypodermis, the 

 outline often becomes somewhat irregular, and more frequently presents 

 the form of a hexagon than of a square (Fig. 59). The number of facets 

 in each eye of an adult lobster is about 13,500. 



