266 DR WALTER H. GASKELL. 



vision a single lens is a necessity, and so it must also be if, as I 

 suppose, eyes existed with an inverted compound retina. Owing 

 to the Crustacean affinities of such eyes, a lens would be formed, 

 and the retina would be compound : owing to the Arachnid 

 affinities, the retina would be inverted and the hypodermal cells 

 which formed the lens would be massed together to form a 

 single lens, instead of being collected in groups of four to form 

 a series of crystalline cones. 



The origin of the sclera and of the cornea, as deduced from 

 the study of Ammocoetes, presents matters of great interest, but 

 as this question is more intimately connected with the origin of 

 the external covering of Ammoccetes, I will not deal with it here, 

 but leave it to the part in which I propose to consider the 

 evidence given by the structure of the skin of Ammocoetes. 



To sum up : The study of the vertebrate eyes, both median 

 and lateral, leads to most important conclusions as to the 

 origin of the Vertebrates, for they show clearly that whereas, 

 as pointed out throughout these papers, the ancestor possessed 

 distinct Arachnid characteristics, yet that it cannot have been 

 a specialized Arachnid, such as our present-day forms, but rather 

 that it was of a primitive Arachnid type, with distinct Crusta- 

 cean characteristics, an auimal that was both Crustacean and 

 Arachnid, but not yet specialized in either direction, an animal, 

 in fact, of precisely the kind which swarmed in the seas at the 

 time when the Vertebrates first made their appearance. In the 

 opinion of the present day, the ancestral forms of the Crustacea, 

 which were directly derived from the Annelida, may be classed 

 as an hypothetical group the Protostraca, the nearest approach 

 to which is a primitive Phyllopod. " Starting from the Proto- 

 straca, say Korschelt and Heider,^ " according to the present 

 condition of our knowledge, we may, as has been already re- 

 marked, assume three great series of development of the Arthro- 

 podan stock, by the side of which a number of smaller indepen- 

 dent branches have been retained. One of these series leads 

 through the hypothetical primitive Phyllopod to the Crustacea ; 

 the second through the Paheostraca (Trilol)ita, Gigantostraca, 

 Xiphosura) to the Arachnida ; the third through forms resem- 

 l)ling Peripatus to the Myriapoda and the Insecta. The 



^ Op. cit., part iii. ]). 427. 



