314 LECTURK XV. 



"Regarding Tight 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 present 

 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 moi'e 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." 



LECTURE 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, and have their basal joint sub- 

 trenchant, if working from before backward. The basal joints of 

 the four following pairs are beset with spines, and act as " carders : " 

 that of the sixth pair is hard, subtrenchant, and works from behind 

 forward : a pair of subquadrate compressed plates, with spiny bor- 

 ders, working in the same direction and closing the mouth below, 

 may represent a rudimental seventh pair of limbs, or a divided 

 labium. In Asaphiis platycephalus Mr. Stokes* discovered a dis- 

 tinct subquadrate labrum deeply emarginate anteriorly ; the nearest 

 approach to this, the only known part of the trophi of the Trilobites, 

 seems to be made by the entomostracous genus Apus, in which, how- 

 ever, the Librum is truncated. A few of the lowest organised Crus- 

 tacea, as Caligus, Nymphon, and Pi/cnogonon, obtain their aliment, 

 like the Epizoa, by suction. 



* Geol. Trans. N. S. vol. i. pi. 27. 



