198 THE PINEAL ORGAN 



system of the Cyclostomes may be considered as a vestigial apparatus, 

 which consists of an imperfectly developed pair of median eyes of the 

 upright type, of which one, the left, is more degenerate than the other, 

 and has been displaced beneath the other less degenerate right organ. 

 Since the latter is placed immediately beneath a transparent " cornea " and 

 has a retina provided with sensory cells, the inner ends of which are 

 connected with the dendrites of ganglion cells, and by nerve-fibres 

 arising from these with the habenular ganglia and posterior commissure ; 

 the right organ may be regarded as capable of receiving impressions of 

 light and transmitting these to the central nervous system. But owing 

 to the imperfect development of the dioptric system no distinct image 

 of an object on the retina is possible. It may be conjectured, however, 

 that in the ancestors of the Cyclostomes both eyes were equally developed, 

 were structurally more highly differentiated and reached to the surface 

 of the body. The absence of a cuticular or epidermal lens in the living 

 representatives of the class may be accounted for by the withdrawal of 

 the eyes from the surface. This supposition being granted, the structure 

 of the pineal eye does not differ very greatly or essentially from the 

 upright median eyes of certain invertebrates. 



The development of the lateral eyes of Petromyzon is similar to that 

 of vertebrates in general ; an optic cup being formed by inversion of the 

 distal part of the primary optic vesicle to form the sensitive part of the 

 retina, while the proximal segment gives rise to the outer or ensheathing 

 layer of the cup and becomes pigmented. The lens also is derived from 

 the cutaneous ectoderm in the usual manner, a hollow vesicle becoming 

 separated from the superficial epithelium by an ingrowth of mesoderm 

 (Fig. 252, Chap. 24, p. 369). 



Variations of the Pineal System in the Different Orders 

 of the Class Cyclostomata 



It is unnecessary to give a detailed description of the variations 

 which occur among the different species of Petromyzon. It is of interest, 

 however, to note that in Mordacia mordax, the pineal organ of which was 

 described by Spencer in 1890, he found no trace of a parapineal organ. 

 Also in the hag-fishes, belonging to the Order Myxinoidei, although several 

 authors have described a single vestigial diverticulum without any 

 thickening of the lens, or pigment in the retina, there seems some doubt as 

 to whether these outgrowths were true pineal diverticula, and according to 

 Kuppfer, who carefully examined a series of specimens in 1900, the parietal 

 region behind the velum is quite smooth and he found no trace of an 

 epiphysis. Both Myxine and Bdellostoma are said to be blind, to a large 

 extent parasitic and of degraded type. Like the lampreys, they feed on 



