508 THE EYE OF THE TUNTCATA. 



the brain, and a retinal portion forming part of the wall of the 

 brain. The refractive parts consist of a convex-concave menis- 

 cus in front, and a spherical lens behind, adjoining the concave 

 side of the meniscus. The posterior part of this lens is im- 



FIG. 296. LARVA OF ASCIDIA MENTULA. (From Gegenbaur ; after Kupffer.) 

 Only the anterior part of the tail is represented. 



N' '. anterior swelling of neural tube; N. anterior swelling of spinal portion of 

 neural tube ; n. hinder part of neural tube ; ch. notochord ; K. branchial region of 

 alimentary tract; d. O3sophageal and gastric region of alimentary tract; O. eye; 

 a. otolith ; o. mouth ; s. papilla for attachment. 



bedded in a layer of pigment. The retina is formed of columnar 

 cells, with their inner ends imbedded in the pigment which 

 encloses the posterior part of the lens. The retinal part of the 

 eye arises in the first instance as a prominence of the wall of 

 the cerebral vesicle: its cells become very columnar and pig- 

 mented at their inner extremities (fig. 8, V, a). The lens is 

 developed at a later period, after the larva has become hatched, 

 but the mode of its formation has not been made out. 



General considerations on the Eye of the Cliordata. 



There can be but little doubt that the eye of the Tunicata belongs to the 

 same phylum as that of the true Vertebrata, different as the two eyes are. 

 The same may also be said with reference to the degenerate and very 

 rudimentary eye of Amphioxus. 



The peculiarity of the eye of all the Chordata consists in the retina being 

 developed from part of the wall of the brain. How is this remarkable feature 

 of the eye of the Chordata to be explained ? 



Lankester, interpreting the eye in the light of the Tunicata, has made 

 the interesting suggestion 1 "that the original Vertebrate must have been a 

 transparent animal, and had an eye or pair of eyes inside the brain, like that 

 of the Ascidian Tadpole." 



1 Degeneration, London, 1880, p. 49. 



