234 



HISTOLOGY 



As has been indicated, a common covering of a simple ciliated epithe- 

 lium covers all the ocelli. It is roughly divided into fields by the slight 

 curvature of the outer ends of the ocelli. 



Below, the ocellus is set into the pigment cup with its lower or visual 

 cap of cells lying in direct contact with the pigment cells. Nerve fibers 

 come up from the sub-dermal nerve layer and provide the ocelli with 

 fibrils. The eyes are evidently very efficiently coordinated with the 

 nerve centers, because, if the light is interrupted from any quarter, the 

 animal at once moves all its wicked spines so that they point to this 

 quarter and are ready to repel an attack. 



nv. 



FIG. 206. Vertical section through an eye-spot of Diadema. Slightly schematic, nv., nerve 

 layer which sends branches to eye through the basal connective-tissue layer (bl.) ; p.c., 

 proximal layer of sensory cells. The three eye units to the left show pigment caps on bases. 

 Those on the right are complete median section. The two in the middle are lateral surface 

 views to show the outer and inner cell caps. (After C. F. and P. B. SARASIN.) 



Although they are far more specialized in most ways, the Arthropods 

 show a type of eye that should be studied at this point on account of 

 structural characters which lead one naturally to think of them when 

 studying the eye of Diadema. This eye as well as Diadema's is called 

 compound because it is composed of a number of similar units, each of 

 which is apparently independent of the others so far as seeing is con- 

 cerned. When the nerve centers with which this eye is connected are 

 studied, however, it is apparent that the eye must act as a whole in 

 some manner. 



A characteristic feature of this eye is the way in which the cuticle 

 is carried into its formation. The cuticle seems to be a more predomi- 

 nant feature in this group than in any other. 



We shall study the eye of Periplaneta orientalis as a type of the ar- 

 thropod eye, referring to a crustacean form, Palcemon squilla, for occa- 

 sional comparison. The outer surface of these eyes shows a rather 



