VERTEBRATES PROM A CRUSTACEAN-LIKE ANCESTOR. 435 



strands of tissue similar to what he has noticed iu the cavity of 

 the dorsal eye. A description of it is given in Owsjannikow's 

 paper (7), and he gives it the name of ventral eye, in contra- 

 distinction to the larger, more perfect organ which is the dorsal 

 eye. This second eye is not only perfectly plain in all my 

 sections, especially the horizontal ones, but it is clearly, as 

 Ahlborn has pointed out, connected in a peculiar w^ay with the 

 left ganglion habenulae : it is built up of similar elements to the 

 dorsal eye, except that it is never pigmented, as far as I have 

 seen, and it is not connected with the cuticular walls of the 

 brain-case. As yet I have not distinguished any rhabdites in 

 it, and the terminations of its nerve- end cells have broken down 

 to such an extent as to give the whole organ the appearance of 

 a tube in connection with the left ganglion habenulse. 



Owsjannikow (7) describes its posterior retinal wall as being 

 connected by means of nerve-fibres with a group of nerve-cells, 

 to which he gives the name of the ganglion of the eye. This 

 group of nerve-cells seen in fig. 21, PI. XXVII {ghl.^.), with 

 the strands of fibres which proceed from them towards the long 

 nerve-end cells of this eye, is called by Ahlborn the '' Zirbel- 

 polster," and is recognised by him as a part of the left ganglion 

 habenulse [ghl^. in his figures). This outlying part of the left 

 ganglion habenulse is united with the rest of that ganglion {ghl-^. 

 of Ahlborn) by a nervous stalk {ghh. of Ahlborn) , which accord- 

 ing to Ahlborn is short in Ammocoetes, but longer in the adult 

 Petromyzon; this connection can be easily followed in a 

 series of transverse sections ; it cannot be shown in any one 

 horizontal section, and therefore is not visible in fig. 21. In 

 Ammocoetes it is so thick that undoubtedly Ahlborn's ghl^. 

 and ghl^. are simply parts of the same left ganglion habenulae. 



Ahlborn further describes how a fold of pia mater entirely 

 separates this outlying part of the left ganglion habenulse 

 from the lower vesicle, i.e. from the ventral eye, except at 

 one place where the continuity of the membrane is broken by 

 the passage of a bundle of fibres connecting the two struc- 

 tures. My sections show clearly the same appearances as 

 those of Ahlborn, and in fig. 21 we see how the fold of pia 



