440 F. R. Cowper Reed^Blind TriloUtes. 



lias been drawn, perhaps too hastily, that we have an example of 

 the operation of causes similar to those which have led to the 

 atrophy of the organs of sight in these modern forms. It is, how- 

 ever, possible to offer two different explanations of the phenomenon 

 of blindness in the case of the trilobites; and it maybe that the same 

 explanation is not applicable to all. In the first place we may regard 

 the absence of eyes as a result of functional disuse, and consequently 

 as an adaptive character suggestive of the environment or mode of 

 life, but of no phylogenetic importance. Owing to the attention 

 which is now directed to the conditions of life, particularly 

 to those of the deep sea, this view finds much favour. The 

 alternative explanation is that the absence of eyes is a morpho- 

 logical feature of ontogenetic or phylogenetic significance, and 

 consequently must have due weight attached to it in any natural 

 system of classification. We shall see that the evidence derived 

 from the development of various trilobites as well as of the whole 

 group points to this being the true explanation in the majority 

 of cases. 



Nature and Position of the Eyes of Trilobites. 



It should first be noticed that the visual organs found in trilobites 

 do not all belong to the same category, for they fall into two classes. 

 There are the so-called ' simple ' eyes or ocelli, and the so-called 

 ' compound ' eyes. They are of widely different origin and are not 

 homologous structures. It is unnecessary here to describe in detail 

 their peculiarities and modifications, except in so far as they affect 

 the question under consideration. 



The most important differences as regards their comparative 

 morphology which we have to remember are that the compound 

 eyes are always borne on the free cheeks and lie on the line of the 

 facial sutures, while the simple eyes (which may more conveniently 

 be termed eye-spots) occur on the fixed cheeks without any con- 

 nection or relation to the facial sutures. It is also possible that 

 visual organs of the same nature as these eye-spots occur on the 

 glabella of some forms. There is a general but superficial 

 resemblance in the position of both these kinds of visual organs 

 from the fact of their being on the lateral portions of the head- 

 shield and from the presence of a pair in each case ; but these are 

 merely accidental analogies connected with their function and with 

 bilateral symmetry. 



The distribution of these two kinds of eyes is of considerable 

 importance and interest to us in our present inquiry, and while 

 much stress has been rightly laid on it by many palaeontologists, 

 yet it is possible to differ from their conclusions. In the case of 

 eye-spots we find them existing as a single pair in the larvae 

 of some trilobites which when adult possess no visual oi'gans 

 (Trinucleus) ; or we find them persisting through life {Harpes). 

 Certain tubercles on the fixed cheeks of other forms may have 

 a similar function, but of this we have at present no proof; and as 

 far as we know paired eye-spots on the fixed cheeks and compound 



