BDELLOSTOMA DOMBEYL LAC. 135 



the largest of the seven. The others decrease in size fi'oni 

 within outwards. The median plate is charaeterized by its 

 larger size, and the possession of a terminal knob at its 

 anterior end. This knob is continued forward for some distance 

 along the roof of the nasal tube as a continually decreasing 

 ridse. So far as my investigations have gone, the olfactorv 

 nerves end in Bdellostoma, as they do in Petromyzon, on 

 either side of this median raphe, so that we now know that all 

 Craniate vertebrates possess bilaterally symmetrical special 

 sense organs since neither group of the Cyclostome forms an 

 exception to the rule. 



The eyes in Bdellostoma have been considered greatly 

 degenerated, and this view is still generally accepted on the 

 strength of Johannes Muller's view of their condition. I feel 

 assured, from a preliminary examination of the eyes, — and 

 my examination is more extended than the one upon w^hich 

 Miiller based his conclusion,^ — that the Bdellostoma eye really 

 represents a phylogenetic stage in the differentiation of the 

 vertebrate eye. Two striking features of this eye are the lack 

 of visible pigment in the retina when viewed from the outside, 

 and the absence of a genuine cornea. From a study of the 

 adult condition, it is highly probable that the skin of the body 

 is never drawn into the formation of the eye as we meet with 

 it in the fishes ; conseciuently, neither crystalline lens nor 

 cornea are present. The original optic cup budded out from 

 the arowino: brain wall forms all there is of this creature's 

 eyeball, and hence it stands as an important and extremely 

 interesting intermediate stage between the Branchiostoma 

 condition and that obtaining among the rest of the sub-kingdom. 

 The large transparent oval spot over each eye serves as a 

 cornea, permitting the light to enter the eye no matter how 

 much the skin may be moved out of its ordinary position. 

 The eyes, along with the rest of the body beneath the skin, 

 are constantly bathed in the lymph which occupies the space 

 between the skin and the muscles. 



1 Kohl (Zoolog. Beitrage) has given us an account of the histology of the eye in 

 Myxine glutinosa and he adheres still to the old view of the degeneracy of this 

 organ. 



