PISCES 



181 



peduncle which is articulated to the skull and to the eye 

 by a knob-like projection of the sclerotic. As in all Craniates, 

 the rods and cones of the retina lie close to the pigment layer ; 

 that is to say, the rays of light have to pass through the nervous 

 layers to reach them. This goes to show that the retinal patch 

 was originally external or developed directly by a pouch-like 

 invagination of the ectoderm. In the Craniates the patch is 

 developed from the forebrain, and its genesis can be seen even 

 before the folding up is accomplished. It still develops' the 

 retinal elements on its original external surface. The eyes are, 

 as in the higher Craniates, the organs of vision and, as in aquatic 



Conjunctiva 

 Cornea 



Iris 



Optic nerve 



Sder. | Ret. \ Ciliary muscle Skin 

 Chor. Lens 



FIG. 91. Raia. Diagrammatic vertical section of eye. 



Craniates universally, the lens is characteristically spherical. 

 In fishes the eyes enjoy a remarkable independence of move- 

 ment, one eye remaining at rest while the other follows the 

 movements of the object watched. 



The auditory organ is developed from the ectoderm, the 

 ganglion and the organ arising in close association. The 

 thickening of the ectoderm deepens to form a paired cup which 

 expands into a large sac connected by a narrow tube with the 

 ectoderm, and lying one on each side of the rhombencephalon 

 (fig. 79). The sac is then modified to form flattened regions 

 mutually at right angles and all communicating with the re- 

 maining wide part of the vesicle. The compression continues 

 until the cavity in each of the three areas is reduced to a 

 marginal canal. Ultimately the parts of the compressed areas 

 which intervene between the canals and the chamber below 



