VERTEBRATES FROM A ORUSTACEAN-LIKE ANCESTOR. 421 



two masses of nervous matter are connected^ as Ahlborn has 

 shown, with each other by a band of fibres forming the com- 

 missura tenuissima, and with the cerebral lobes by strands 

 of peculiarly coloured fibres which he calls the taenia thalamic 

 represented in fig. 8, PI. XXV, by t. th. That part of the 

 nervous system connecting together the cerebral lobes and the 

 ganglia habenulseis recognised by Ahlborn as the optic thalami 

 {tho-. in fig. 8)./^,"Z^ 



Sect. 8. — The Median Eyes and their Optic Ganglia. 



In close connection with the ganglia habenulae are the 

 structures known as the dorsal and ventral pineal eyes. Of 

 these the dorsal eye is the large and conspicuous intensely white 

 object which is seen in front of the right ganglion habenulse ; 

 and the descriptions which have been given of the pineal eye 

 refer in the majority of instances to this eye, and not to the 

 much more insignificant ventral eye. Before entering upon a 

 criticism of the statements which have been already published 

 respecting the pineal eye, I will describe the appearances pre- 

 sented by my sections which have forced me to the conclusion 

 that this eye is, as I have already stated in my paper in ' Brain ' 

 (2), an Invertebrate eye of the Crustacean or Arachnidian 

 type. 



I have cut sections through a large number of heads of 

 Ammocoetes of sizes varying from 25 to over 100 mm. 

 These sections have been cut either transversely to the long 

 axis of the animal or horizontally in the direction of that 

 axis, and I have found that the transverse sections do not cut 

 the eye in the direction of its optic axis, but that the hori- 

 zontal sections give a much nearer approximation to sections 

 parallel to the true median plane of the eye, so that we must 

 imagine the eye has been bent forwards so that its optic axis is 

 directed somewhat forwards as well as upwards. Also by com- 

 pression the eye has been distorted out of shape sufficiently to 

 prevent any plane of section accurately passing through the 

 median plane of both eye and nerve. In figs. 20 « — d, PI. 

 XXVII, I give selections only of a series of horizontal sections 



