Evolution of Fishes from Invertebrates 8i 



place between the lateral nerves, and so such fibres establish 

 a commencing condensation and correlation-relation between 

 these lateral nerves, that must aid in their subsequent 

 approximation and union. This correlation and anastom- 

 osis is further advanced in chordate animals. 



X. Structure of the sense-organs. 



The external aspect and relation of the sense-organs 

 have already been dealt with. As to their minute structure 

 and mode of origin, highly interesting and connecting stages 

 of evolving complexity can be traced, some points of which 

 have been dealt with above, (a) The nasal organ as ac- 

 cepted by the writer for nemerteans, is supplied abundantly 

 by nerves which pass off from the anterior region of the 

 dorsal brain-lobes. "As described and figured by Burger 

 the epithelial cells of each nasal depression consist of cil- 

 iated sensory or olfactory cells, that alternate with inter- 

 stitial or supporting cells, an arrangement exactly similar 

 to that seen in the vertebrate olfactory organ." 



(b) The eyes in higher nemerteans show a minute struc- 

 ture and sensory complexity that closely allies them in 

 ascending series with the eyes of vertebrates. Nerves pass 

 to them from a lateral part of the dorsal brain-lobes, that 

 seem exactly to correspond to the optic brain centres in 

 the mid-brain of fishes. Each eye shows a cup-shaped 

 depression composed of columnar retinal cells, a pigment 

 zone, a finely granular cup-substance, an optic capsule, and 

 delicate ganglion cells that pass to and end below the 

 columnar cells. In the young of Petromyzon and in Myxine 

 the eyes are now reduced to two functional on either side 

 of the head, and in the former to two vestigial eyes that are 

 in sub-median or median dorsal position on the head, these 

 being the pineal and parapineal or parietal eyes of authors. 

 The functional ones then are scarcely more complex than 

 in higher nemerteans, being still devoid of lens, cornea, and 

 sclerotic constituents, as well as eye-muscles. But in the 

 adult Petromyzon these have all been formed, and are con- 

 tinued in higher fishes, where such additional complexities 

 as a choroid gland, a cartilaginous or osseous sclerotic lay- 

 er, a retinal tapetum lucidum, etc., arise 



