THE PINEAL SYSTEM OF VERTEBRATES 185 



The parietal sense-organ in those animals in which it is best developed, 

 such as in the Cyclostomes and certain lizards, consists of a hollow 

 vesicle, the superficial wall of which is lens-like, while the proximal 

 part is differentiated as a retina of the upright type, having receptive 

 sensory cells, pigment cells, and ganglion cells, with nerve-fibres pro- 

 ceeding from these to form a parietal or pineal nerve which terminates 

 directly in the habenular ganglion of the same side or in either the 

 habenular or posterior commissure, or in both of these and so reaches 

 the habenular ganglia and other nerve centres with which the commissures 



A 

 Fig. 133. — Dorsal View of the Brain of Petromyzon marinus, showing the 

 Right Pineal Organ and Large Right Habenular Ganglion. (After 

 Ahlborn.) 

 Cbl : cerebellum ; CN. I. : cranial nerve I. ; F. Rh. : fossa rhomboidalis ; 

 M. Br. : midbrain ; JR. Pi. O. : right pineal organ. 



are connected. The nerve-fibres may pass by the stalk of the parietal organ 

 as in Petromyzon, or they may course independently of the stalk, being 

 formed secondarily as in Lacerta (Figs. 172, D, 173, Chap. 20, pp. 243, 244). 



The parietal sense-organ may be cut off and separated from the pedicle 

 by the growth of the vault of the skull (Figs. 161, 162, Chap. 19, p. 228). 

 In these cases it may persist throughout life as a functionless organ lying 

 beneath the skin as in the frog, in which animal it remains as the " frontal 

 organ " of Stieda, or it may fail to develop. In both cases the base of the 

 stalk and other parts may persist as the pineal body (pineal organ ; pineal 

 sac ; pineal gland ; conarium ; epiphysis), the detailed structure of which 

 will be considered later. 



B. The parapineal organ : this is the term which was applied by 

 Studnicka to the anterior smaller end-vesicle Epiphysis I of Petromyzon 

 in his description of the pineal system of Cyclostomes. In reptiles it is 

 represented according to Studnicka by the anterior of the two organs, 

 which in the Reptilia is the more fully developed organ and becomes the 



