770 



CRUSTACEA. 



immediately into contact with the optic nerve. 

 These eyes are obviously made up by the con- 

 junction of several stemmata under a common 

 cornea. The Apus, besides its pair of simple 

 eyes, presents another compound pair, behind 

 and at some distance from these. 



The Amphihoe Prevostii and some other 

 Edriophthalmians present the transition from 

 the form last described to that of truly com- 

 pound eyes, having distinct facets. The cornea 

 in these is formed of two transparent laminae, 

 the external of which is smooth and without 

 divisions, whilst the internal is divided into 

 a variable number of hexagonal facets, each 

 of which is a distinct cornea, superposed upon 

 such a conical crystalline lens, as we shall 

 have occasion immediately to describe when 

 speaking of compound eyes properly so called, 

 or eyes with simple facets. 



In these the two membranes, external and 

 internal, the union of which constitutes the 

 cornea, present simultaneously the division 

 into facets, each of which forms anteriorly an 

 ocular compartment proper to it. These facets, 

 always hexagonal in insects, are of various 

 forms in the Crustacea : thus in the Astacus 

 fluviatilis, the Peneae, the Galatheae, and 

 the Scyllari, they are square (fig. 399), whilst 

 the Paguri, the Phyllosoma, the 

 Fig. 399. Squillae, the Gebiae, the Calli- 

 anassae, and the Crabs, have them 

 hexagonal (fg. 400). The crys- 

 talline that succeeds them imme- 

 diately is of a conical form, and 

 is followed by a vitreous humour 

 Fig. 400. having the appearance of a gelati- 

 nous filament, adhering by its base 

 to the optic nerve. Each of the 

 columns thus formed is, more- 

 over, lodged within a pigmentary 

 cell, which likewise covers the 

 bulb of the optic nerve. But 

 the most remarkable circumstance is, that 

 the large cavity within which the whole of 

 these parallel columns, every one of which 

 is in itself a perfect eye, are contained, is 

 closed posteriorly by a membrane, which ap- 

 pears to be neither more nor less than the 

 middle tegumentary membrane, pierced for 

 the passage of the optic nerve ; so that the 

 ocular chamber at large results from the sepa- 

 ration at a point of the two external layers 

 of the general envelope. 



Fig. 401. 



Longitudinal section of the Eye of the Lobster. 



The gelatinous or vitreous elongated pro- 

 cesses which succeed the conical crystallines 

 have been looked upon by several anatomists 

 as ramifications of the optic nerve ; but we 

 do not imagine that they are so in reality. 

 In the Lobster, for instance, we have even 

 seen the surface of the bulb isolated from the 

 masses in question, divided into compartments. 



Fig. 402. 



corresponding to those of the cornea itself, 

 and lined with a layer of pigmentum perfectly 

 distinct. 



The most remarkable modification of facetted 

 eyes consists in the presence of a kind of sup- 

 plementary lens, of a circular shape and set 

 within the cornea in front of each proper crys- 

 talline lens (fg. 402). These small lenticular 

 bodies exist independently, and 

 are perfectly distinct from the 

 small corneal facets. In some 

 cases they might be mistaken 

 (in the Idoteae, for example, 

 where they may be perceived 

 singly, and with their distinct 

 circular forms), and the incau- 

 tious observer led to conclude that the cor- 

 neal facets are merely these lenticular bodies 

 so much enlarged that their hexagonal or 

 square forms result from their agglomeration 

 in a point; but there are Crustacea, such as 

 the Callianassse, in which these two elements 

 of the external cornea may be perfectly dis- 

 tinguished, the lenticular body being of insig- 

 nificant dimensions and occupying the centre 

 of the corneal facet only (Jig. 402). In general, 

 howeve'r, the diameter of the lenticular body 

 is equal to that of the corneal facet itself, so 

 that their edges blend. Farther, the lenticular 

 bodies are most commonly evolved in the sub- 

 stance of the cornea; but there are cases in 

 which, under favourable circumstances, they 

 may be detached from it. 



Although the existence of these different 

 modifications must not be understood as being 

 exclusive, inasmuch as there are certain Crus- 

 tacea which exhibit more than one of them at 

 the same time, for instance, stemmata and 

 compound eyes, the latter only are the species 

 of visual organ encountered in the great ma- 

 jority of cases. Their general number is two ; 

 but these are occasionally united, so as to 

 form a single mass, and make the animal 

 appear at first sight as if it had but a single 

 eye. This peculiarity of organization can even 

 be followed in the Daphniae, in the embryo 

 of which the eyes are first seen isolated ; with 

 the progress of the development, however, 

 they are observed gradually to approach each 

 other, and finally to become united. Stemmata 

 are always immoveable and sessile ; the com- 

 pound eyes with smooth corneae, however, 

 although in the majority of cases they present 

 the same disposition, now and then occur 

 moveable : sometimes they are supported by a 

 pedicle, moveable in like manner, and pro- 

 vided with special muscles. The eyes with 

 facets present the same 

 modifications, and even 

 supply important charac- 

 ters in classifying these 

 animals : thus in the 

 Edriophthalmia the eyes 

 are always immoveable 

 and sessile, (fg. 403,) 

 whilst in the Decapo- 

 da and the Stomapoda 

 (fig- 404) they are sup- 

 ported upon moveable 



Fig. 403. 



