F. R. Coivper Reed — Blind TriloUtes. 441 



eyes on the free cheeks never exist together at the same time in the 

 same individual. 



It should also be noticed that the paired eye-spots are generally 

 situated at the outer end of the so-called ' ocular ridges ' which run 

 outwards from the glabella, and though morphological structures 

 which are apparently homologous run out in the same way as ridges 

 to the compound eyes of certain other forms, yet we shall see that 

 this feature is not so anomalous or difficult of explanation as it may 

 at first appear. 



We shall also perceive when we consider the families of the 

 trilobites separately that paired eye-spots are only found amongst 

 those of a low phylogenetic rank, and that compound eyes are 

 characteristic of the higher and more differentiated genera. 



All the principal features of those forms possessing eye-spots are 

 of a primitive type, corresponding to those exhibited in the larval 

 and pre-adult stages of those with compound eyes. Further remarks 

 on this fact will be made later. It may, however, be mentioned 

 here that since no such organs as eye-spots have been found in 

 the larval stages of the higher forms, we must not in the light of 

 present knowledge regard them as larval structures common to the 

 whole group, but rather as separately developed in certain genera 

 possessing a low phylogenetic rank more or less masked by 

 a considerable amount of secondary specialization, just as is the case 

 with the Metatheria in Australasia and in many smaller groups of 

 animals. These lowly organized trilobites, while retaining certain 

 essential primitive characters, have developed other characters 

 analogous in function or even in structure to those present in 

 forms much higher in the scale of evolution. Bearing this idea 

 in mind, we may understand how the possession of visual 

 organs, though inferior in quality to those of the higher trilobites, 

 may have been one of the causes which enabled such a form as 

 Sarpes to maintain so long the struggle for existence. 



That there were other factors at work than the power of sight 

 is evident from Trinucleus losing its eye-spots when mature, and in 

 one species of Harpes being blind. These considerations suggest 

 that the conditions of life have left marks of their influence as well 

 as the phylogenetic rank, and indicate that the same explanation 

 is not applicable to every case. We must therefore examine each 

 on its own merits. 



With regard to the compound eyes of trilobites, their precise 

 position along the line of the facial suture is subject to considerable 

 variation, and their size in relation to that of the wliole head-shield 

 is also variable within wide limits. The very small eyes of such 

 a genus as Acidaspis afford the greatest contrast to those of Gi/clopyge 

 (vS^glina). But while to some extent the development and position 

 of the compound eyes is of phylogenetic importance, yet we shall 

 be able to show that in some cases it is certainly a secondary 

 adaptation and apt to mislead us if regarded in any other way. 

 We can, however, in this article only deal with the subject of the 

 size and position of the compound eyes in so far as it concerns 

 the inquiry into the meaning of blind trilobites. 



