384 



Comparative Animal Physiology 



tain marine dinoflagellates are much larger. In general appearance they re- 

 semble the ocelli of flatworms, and they are called ocelli. 



The ocelli of coelenterates vary considerably in structure, ranging from a 

 simple layer of sensory cells mingled with pigment cells, as in Tunis, to cup- 

 shaped ocelli in Sarsia (Jig. 103, A) and Tiaropsis. 



The ocelh of flatworms are similar in general appearance and in prin- 

 ciple to the stigmas of the Phytomonadida and Dinoflagellida. There is a 

 pigment cup and a sensitive layer within the cup (Fig. 103, B), and some- 

 times a lens. However, the sensitive layer is cellular and is composed of pri- 

 mary neurones, the axons of which go to the brain. The pigment cup may 

 consist of a single cell. 



Some annelida (e.g.. Nereis) have ocelli. Among the Mollusca there exist 

 several tvpes of photoreceptors which transcend in complexity the simple 

 ocelli heretofore discussed. Although the necessary data are not at hand. 



Flagellum 



Eye -spot or 

 pigment shield 



Dorsal or 

 blind side 



Gullet 

 (reservoir) 



Ventral or 

 seeing side 



Fig. 101. Side view of Eiiglena viridis, showing the eye spot or pigmented shield 

 and the enlargement at the base of the flagellum (rec), believed to be the photo- 

 receptor. From Fraenkel and Gunn." 



it is quite possible that the eyes of Nautilus, consisting of an open pit, with- 

 out a lens, the eyes of Helix, consisting of a completely enclosed receptor 

 containing a lens-like material, and the eyes of Pecten (Fig. 103, D), con- 

 sisting of a lens and a double layer of retinal cells, backed by a tapetum, are 

 capable of pattern vision as well as intensity perception. The most com- 

 plicated type of molluscan eye is represented by that of Sepia (Fig. 103, C). 

 Generally speaking, in structure and development this eye is comparable to 

 the eyes of vertebrates. There is a corneal surface which mav be covered by 

 lids; there are an iris and a lens which separate the eye into chambers; the 

 lens is suspended and can be moved by a ciliary muscle; the retina lines the 

 back of the eye; and the eyeball is surrounded bv cartilage. During develop- 

 ment the eye originates as an ectodermal pit, the lining of which forms the 

 retina. The lens is formed in two parts: the inner part at the point of 

 closure of the pit, and the outer part from a circular fold which forms the 



