174 LECTURE XIV. 



closed by a membrane, excavated in the first joint of the second pair 

 of antennae, in the lobster and other Macroiira. In most of the 

 Brachiura the membrane is stated by Dr. Edwards to be replaced by 

 a small moveable calcareous disc, which is pierced with a small oval 

 opening, over which there is stretched a thin and elastic membrane. 

 The external opening of the ear is closed by this bony disc, A 

 second small plate is so situated as to regulate the tension of the 

 auditory membrane, whilst the rigid stem of the antennae, in which 

 the whole organ is situated, is well adapted to render the auditory 

 vibrations more distinctly perceptible. These vibrations are con- 

 veyed through the medium of a vesicle filled with fluid to a branch 

 of the antennal nerve which expands in the vesicle. 



With respect to the organ of vision, we find in the class Crustacea a 

 most extensive and interesting series of gradations, leading from the 

 sessile median eye-speck to two distinct eyes, provided with all the 

 essential optical apparatus and placed upon moveable peduncles. Ocelli 

 or stemmata are combined with compound eyes 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 compound 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 compound 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 re- 

 spective ocelli, of the aggregate of which the large lateral eye is com- 

 posed : 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 the sessile eyes of the Edriophthalma^ as, for example, in the 

 Serolis, the inner layer only of the cornea is divided into hexagonal 

 facets, corresponding with the number of the conical crystalline 

 lenses of the compound eye. But in the Trilobites, the cornea pre- 

 sents the same subdivided surface as in the Limulus ; and the position 

 of the two eyes agrees with that of the corresponding compound pair 



