BDELLOSTOMA DOMBEYI, LAC. 151 



by a median plate which is the homologue of the median raphe 

 of Petromyzon. In Figs. 7, 8 is shown this important feature 

 as seen from the ventral surface (inner face) of the nasal organ. 

 As concerns Bdellostoma's eye, we know very little of its adult 

 structure and nothing of its development, and the conclusion 

 that it is a degenerate organ in lacking a lens, choroid, etc., 

 and being entirely below the surface of the body and under the 

 skin, is purely hypothetical, for it can just as well represent 

 a stage in the phylogenetic development of vertebrates. I 

 believe such to be the truth. The undegenerate nature of 

 the ear I have already established in another publication. 1 On 

 the basis of our knowledge of the higher sense organs we may 

 well ask: Why are the Myxinoids primitive fishes? instead of 

 "Why are they degenerate vertebrates ? " The following con- 

 siderations enable us to understand why they are primitive and 

 not degenerate: 



1 . Their unusually great geographical distribution indicates 

 that they are descendants of a very ancient stock. 



2. They are in a morphologically undifferentiated condition, 

 as compared with all other groups of craniate vertebrates and 

 their several organs show no certain traces of degradation 

 of structure. In this regard I have examined the following 

 organs : skin (here the absence of surface sense organs, scales, 

 except the teeth, and a lateral line is noteworthy and entirely 

 unexplained; they are not degraded but Jiave entirely vanished 

 from adult life], muscular system, skeleton, vascular system, 

 alimentary system, urino-genital system and the higher sense 

 organs, eye, ear and nose. Of these latter the two last are 

 normal stages in the development of the vertebrate stock, 

 and so far as I have examined the structure of the eyeball, 

 its lack of lens and eye muscles and its small optic nerve, 

 I have found no conditions which are not harmonious with the 

 view that this eye has simply never been highly developed. 



The degeneration hypothesis fails completely to account for 

 Myxinoid anatomy, and the habits of the animal give no 

 grounds for this assumption. Certainly the natural conditions 

 surrounding the life of the animal as far as we know them, are 



1 Contributions to the Morphology of the Ear, Jonrn. ^^orph^.>L VII, 1892. 



