STRUCTURE OF THE AMAROUCIUM TADPOLE 81 



ever, one or more granules are usually found. The lenses are 

 easily sectioned and are not crystalline. 



The nuclei of the lens-forming cells lose their staining quali- 

 ties when the lenses have been fully formed and the shrunken 

 cell bodies persist merely as anchors by which the lenses are 

 attached to the superior wall of the sensory vesicle (compare 

 figs. 4, 6, and 11). 



The retina consists of a layer of large nerve cells grouped 

 about the pigment cup. In the axis of some and possibly of 

 all of the retinal cells a rod-shaped portion is differentiated 

 which penetrates the pigment zone and ends at the inner sur- 

 face of the pigment cup. These portions of the retinal cells 

 may be termed the visual rods. They are so placed in the zone 

 of pigment that their long axes coincide with the direction taken 

 by rays of light focused by the lenses into the pigment cup. 

 A visual rod is shown in longitudinal section in figure 11 and four 

 in transverse section are shown in figure 10. 



An optic nerve, such as that described by Salensky ('93) in 

 the larva of Distaplia, connecting the retinal portion of the 

 sensory vesicle with the visceral gangUon, I have failed to find, 

 but, as these two parts of the central nervous system of the 

 Amaroucium tadpole are practically in contact, the visceral 

 ganghon probably receives the retinal fibers directly. 



The eye of the ascidian tadpole is a true direct brain eye and, 

 as has been pointed out first by Goette (75) and later by Salensky 

 ('93), Willey ('94), and others, it is similar in its structure and 

 organization to that of the pineal or parietal eye of cyclostomes 

 and Hzards. McBride ('14), on the other hand, possibly with 

 the observations of Lahille in mind, commits himself to the 

 view that the eye of the ascidian larva is homologous with one 

 of the paired lateral eyes of vertebrates. Lahille ('90) described 

 what he interpreted to be the remains of an atrophied eye be- 

 longing to the right side of the sensory vesicle of the larva of 

 DistapUa, but Salensky ('93) and others, working with the same 

 larva, have failed to find any trace of the rudimentary structure 

 described by Lahille. 



