VERTEBRATES FROM A CRUSTAOEAN-LIKE ANCESTOR. 431 



eye itself arises the nerve, which is most beautifully shown in 

 the series from which figs. 20a — d are taken. It is seen 

 to be a delicate thin nerve, which is not mixed up with any 

 other structures, and passes freely from the eye to the 

 large ganglion habenulee. It is clearly not hollow, and at the 

 same time is filled with something which does not look like 

 nerve-fibres, but more like somewhat elongated cells closely 

 packed end to end. The only place where there is any appear- 

 ance of a cavity is where the central cavity is prolonged in 

 the direction of the place of entrance of the nerve. This con- 

 tains the remains of nerve-end cells and rhabdites, just as the 

 rest of the central cavity. 



The shape of the central cavity is given in Ahlborn's fig. 44 

 (3), which represents a sagittal section through the eye. Its 

 shape is due to the arrangement of the structural elements of 

 the eye, and resembles the picture of the eye of the larva of 

 Hydrophilus given in Patten's paper (25). Horizontal sections 

 show that the axis of the cavity has been twisted upwards in 

 the dorsal direction at the end Avhere the nerve enters, so that 

 the shape of the cavity may be likened to a cornucopia, the 

 bent part of the stalk of which lies in the dorso-ventral plane. 

 As the result of this distortion all the most dorsal of a series 

 of horizontal sections must cut through the central cavity 

 twice, first through the expanded mouth of the cornucopia, 

 then through the stalk end, and in these sections the entrance 

 of the nerve must be shown (see figs. 20a, 20^, PI. XXVII); 

 then in the middle sections one cavity will be shown consisting 

 of a section through the expanded mouth and commencing stalk 

 of the cornucopia (fig. 20 c, PI. XXYII), and finally on the 

 ventral side of the middle section one cavity only will be shown, 

 viz. the cavity of the expanded mouth of the cornucopia (fig. 

 20^, PI. XXVII). 



In this way the appearances seen on section receive a 

 simple explanation, and in all probability the small circular 

 cavity depicted by Owsjannikow in figs. 7, 8, and 9 of his 

 paper (7) is due to the direction of his sections in a preciselv 

 similar manner. 



