258 ORGANS OF THE SENSES. 



of these lenses, where the optic nerves terminate, as usual, in 

 their small soft retinal expansion. The optic nerves through- 

 out their course in the eye-ball are enveloped in the same 

 dark pigment which coats the lenses. The lenses in the 

 cray-fish and in the palcemon sulcatus, are not round in their 

 outline, but four-sided truncated pyramids ; most generally, 

 however, they are regular smooth cones truncated at their 

 interior apex, and they are seen of this form even under the 

 smooth and undivided corneas spread over the eyes of mono- 

 culi. In the compound eyes of the branchipus stagnalis, how- 

 ever, it has been recently observed that, behind the smooth 

 surface of the corneas, there are distinct round or ovate 

 lenses supported each on the anterior end of an elongated 

 vitreous humour. This vitreous body tapers backwards to 

 the optic nerve, it is enclosed in a hyaloid membrane which 

 embraces also the posterior half of the lens, and it is entirely 

 covered externally by a retinal expansion of the optic nerve 

 as far as the middle of the lens ; thus presenting a structure 

 similar to that lately detected in the compound eyes of several 

 insects. There are about five thousand eyes in the two com- 

 pound organs of the lobster. The optic nerves in the 

 Crustacea, as in insects, enlarge into an optic ganglion 

 within the globe of the compound eye, from which ganglion 

 the small filaments radiate outwards to the separate lenses 

 of the component eyes, and each minute eye appears to have 

 a pupilar extension of the choroid around the anterior surface 

 of its lens. Thus the compound eyes of the Crustacea and of 

 insects are but repetitions of the simple organs of the leech 

 and the planaria, and probably of the ocular papillae of the 

 hydatina, the medusa and the monad. The mobility so re- 

 markable in the pedunculated eyes of the decapods is already 

 perceptible in the elevation and retraction of the isolated 

 organs of the annelides, and this power of varying the direc- 

 tion of the organs is greatly en creased in the pedunculated 

 eyes of mollusca and even of some fishes. In the great deve- 

 lopment of the cornea and the lens throughout the articulated 

 classes, we observe the early perfection of the most dense 

 and refractive parts, on which the optical properties of this 

 organ chiefly depend. 



The organs of vision are less required and less generally 

 developed in the slow moving or fixed molluscous animals 



