176 LECTURE XV. 



light to organs of vision, the nature of which is so fully disclosed by 

 the state of perfection in which they are preserved. With regard to 

 the atmosphere, also, we infer that had it differed materially from its 

 actual condition, it might have so far affected the rays of light, that 

 a corresponding difference from the eyes of existing Crustaceans 

 would have been found in the organs on which the impressions of 

 such rays were then received. 



" Regarding light itself, also^ we learn, from the resemblance of 

 these most ancient organisations to existing eyes, that the mutual 

 relations of light to the eye, and of the eye to light, were the same 

 at the time when Crustaceans, endowed with the faculty of vision, 

 were first placed at the bottom of the primeval seas, as at the pre- 

 sent moment. 



" Thus we find among the earliest organic remains, an optical in- 

 strument of most curious construction, adapted to produce vision of 

 a peculiar kind, in the then existing representatives of one great class 

 in the articulated division of the Animal Kingdom. We do not find 

 this instrument passing onwards, as it were, through a series of ex- 

 perimental changes from more simple into more complex forms ; it 

 was created at the very first, in the fulness of perfect adaptation to 

 the uses and condition of the class of creatures to which this kind of 

 eye has ever been, and is still, appropriate." 



LECTUEE XV. 



CRUSTACEA. 



The Limuli, which form the only genus of large Crustaceans repre- 

 sented by species which co-existed with the Trilobites, differ from all 

 other living Crustacea in their organs of mastication, which are the 

 modified basal joints of the five posterior pairs of legs : the first 

 small pair serve to bring the objects of food to the mouth ; they are 

 supported on a rudimental labrum. In the Asaphus platycephalus Mr. 

 Stokes has discovered a distinct subquadrate labrum deeply emar- 

 ginate anteriorly; the nearest approach to this, the only known part 

 of the trophi of the Trilobites, seems to be made by the entomostra- 

 cous genus Apus, in which, however, the labrum is truncated. A 

 few of the lowest organised Crustacea, as Caligus, Nymphon, and 

 Pycnogonoiiy obtain their aliment, like the Epizoa, by suction. 



In the Malacostracous Crustacea the mouth is closed by a small and 



