Vertehrata. 



337 



A peculiar third unpaired eye, the pineal eye (parietal eye), in 

 connection with the pineal body, has been recently described in some Lizards 

 (e.g., in the common Lacerta, Blind-worm, and others). It Ues in a small perfora- 

 tion of the upper wall of the skull (in the parietal bone, or between this and the 

 frontal), close below the skin, which is at this spot partially transparent. It 

 consists of a vesicle formed of a simple epithelium, the upper portion forming a 

 lens-like thickening, whilst the lower part is much pigmented (retina). In 

 other Lizards the same structure occurs in the same position in a more 

 rudimentary form, as a simple vesicle, which does not resemble an eye, but is 

 uupigmented, and without a lens. A structui'e Uke the first desci-ibed occurs 

 also in the Cyclostomi, where, however, it is covered by the skull, which 

 is often somewhat thin at this place, and may even, like the skin above it, be 

 transparent. In various other Yertebrates also, facts are known which point to a 



Fig. 277. Pineal eye of a Lizard; diagii-ammatio. ^ brain and npper wall of the 

 skull, the latter cut through ; B pineal eye alone, in section. F, Z, M, H cerebrum, 

 thalamencephalon, optic lobes, cerebellum ; h skin, s roof of skull, o unpigmented portion 

 of skin below which the pineal eye Kes, in a hole in the roof of the skull ; p epiphysis, 

 t hypophysis, 2 optic nerve. L lens, R retina, N nerve of pineal eye. — Orig. (using 

 Spencer's figures). 



connection between the pineal gland and the exterior. In the Selachians the 

 pineal gland is filiform, and its distal end lies in a hole in the dorsal wall of the 

 skull, covered entirely by skin ; so far as is known it possesses no optic structure. 

 In the A n u r a also, the epiphysis, which is short at first, elongates dm-ing larval 

 life into a long thread, with a, terminal enlargement ; the thread perforates the 

 skuU-wall, and the swelling lies on the upper side of the head, directly beneath 

 the epidermis. 



The auditory organs, one on either side, arise in the embryo 

 as vesicular invaginations of the epidermis, which gradually sink in 

 deeper, and become surrounded by the cartilage of the skull (later often 

 by bone). The invagination remains for some time connected with the 

 exterior by means of a canal, but later is usually completely cut off 

 from the skin, so that the developing organ is a closed sac ; in some 

 cases, however (the Selachians), the canal persists throughout life 

 as an open tube. The vesicle does not retain its primitively simple 

 form, but is modified, so that the auditory organ in its adult condition 



Z 



