312 LECTURE XIV. 



in the same species in certain Entomostracans, as Apus and Limulus. 

 A transparent speck of the integument forms the cornea of the 

 ocellus, immediately behind which there is a spherical crystalline 

 body in contact with a gelatinous or vitreous humour, upon which 

 the extremity of the optic nerve expands : a layer of dark pigmentum 

 covers all these parts with the exception of the cornea. In the com- 

 pound eyes of Daphnia, the smooth undivided cornea protects and 

 transmits the rays of light to an aggregation of small ocelli, each of 

 which is lodged in a pigmental cell : the similarly constructed com- 

 pound eye of the active little Branchipus is supported on a short 

 moveable peduncle. 



The large lateral compound eyes of the Limulus are sessile : the 

 cornea is divided into a considerable number of small circular facets, 

 each of which corresponds to an ocellus ; and the optic nerve, after 

 its long course as a simple chord, divides near the eye into a pencil of 

 fine filaments, which severally receive the impressions from their 

 respective ocelli, of the aggregate of which the large lateral eye is 

 composed. The two small simple median eyes, which are almost in 

 contact, command the space before the head, which is out of the range 

 of the large compound eyes : each simple eye receives its distinct 

 nerve from the anterior apex of the corresponding cerebral lobe. 



In some of the Trilobites, the cornea presents the same subdivided 

 surface as in the Limulus ; and the position of the two eyes agrees 

 with that of the corresponding compound pair in the large existing 

 Entomostracan. The eyes are more elevated in some of the Trilobites, 

 as Phacops. In Asaphus caudatus the cornea is divided into at 

 least 400 compartments, each supporting a circular prominence : its 

 general form is that of the frustum of a cone incomplete towards the 

 middle line of the head, but commanding so much of the horizon in 

 other directions, that where the distinct vision of one eye ceases, that 

 of the other begins. 



In certain Phyllopoda and Amphipoda a common cornea covers 

 numerous closely aggregated simple eyes, consisting each of a py- 

 riform lens, the apex of which is sunk in a black or brown pigment. 

 The optic nerve, before reaching the pigment, divides into as many 

 branches as there are lenses. In the sessile eyes of other Edrioph- 

 thalma, as, for example, in Serolis, the inner layer of the cornea 

 is divided into hexagonal facets, corresponding with the number of 

 the conical crystalline lenses of the aggregate eyes. In Amphithoe 

 this has been regarded as a second distinct cornea. 



In the mandibulate Crustaceans, distinguished by having their 

 compound eyes supported on moveable peduncles, the form of the 

 corneal facets varies ; they are square in the lobster and river craw- 



