442 F. R. Cotcper Reed — Blind TrilohUes. 



The Development of the Compound Eyes of TriloUtes, 



Beeclier ^ has recently brought together and admirably summarized 

 the results of recent researches on the evolution of the head-shield 

 in trilobites, and his conclusions are of much significance to us in 

 our present inquiry. He emphasizes the fact that the most primitive 

 larvEe show no eyes on the dorsal shield and have no free cheeks 

 visible, for the latter are ventral in position and the suture is 

 marginal or submarginal. It has, moreover, been observed that 

 a number of genera possess in the adult condition characters which 

 agree closely with those found in the larvee of other and higher 

 genera. For instance, the mature individuals of such genera as 

 Carausia and Aneucanthus possess the pi'incipal cephalic features of 

 the simple protaspis larval stages of the higher genera Ptychoparia, 

 Solenopleura, and Liostracns ; they show no eyes and no free 

 cheeks, and thus indicate that they belong to a lower grade in the 

 evolution of the group.^ These primitive types predominate in the 

 earlier Palaeozoic faunas, so that their stratigraphical distribution 

 confirms the deduction from their morphological characters. The 

 absence of eyes, therefore, in such primitive forms as these cannot 

 be interpreted as an adaptive character, but must be regarded as 

 a mark of their phylogenetic rank. All the genera which are 

 characteristic of the Cambrian faunas, and the development of 

 which is known, pass through this blind larval stage, and a number 

 of genera {Agnostus, Microdiscus, etc.) never pass beyond this stage, 

 but preserve their primitive features to maturity. 



The next stage in the larval history of the more advanced forms is 

 marked by the appearance of the free cheeks on the upper surface 

 of the head-shield as narrow marginal bands, and by the simple 

 curved course of the facial suture which cuts them off. The 

 genera Ampyx and Conocoryphe, s.str., exhibit these features when 

 adult and have no compound eyes. In genera of a higher rank which 

 possess compound eyes, the latter generally appear on the margin 

 almost simultaneously with the free cheeks which bear them ; but 

 this simultaneous appearance is due to accelerated development and 

 to the operation of the law of earlier inheritance. In later larval 

 stages the free cheeks increase in width and the eyes move inwards. 

 The successive acquisition of the foregoing characters in definite 

 order is more or less indistinct in the higher and more specialized 

 trilobites which attain their maximum in Post-Cambrian times, for 

 there is a considerable acceleration of development, and their 

 larvae consequently exhibit characters which do not exist in the 

 corresponding larval stages of the more ancient and primitive 

 genera, but which only appear in the later larval or even adult 

 stages of the latter. The two principles (1) that ontogeny furnishes 

 the means of recognizing the true affinities of organisms, and (2) that 

 the development of the individual correlates with the development 

 of the group, are so firmly established that it is needless to remark 



1 Amer. Geo!., vol. xvi (1895), p. 166. 



2 Beecher, Amer. Journ. Sci., vol. iii (1897), pp. 89 and 181. 



