ANIMAL PHYSIOLOGY. 



97 



ANIMAL THY V._ III. 



'I Mi: EYE (eondudal). 



TIIK eyes of tho animals lower than fiwli, none of which have 



::., ami which aro oallod invertebrate auimala, aro closely 



ivlutr.I to tln'ir powers of moving from plaoo to place. If an 



ummul can dart rapidly about, moro especially if it can move 



. v.it'tly for h< in i. > tin m at a stretch, iU eyes are usually Tory 



: Imt if it can only crawl sluggiuhly, itH eyes arc of an 



luforior structure. 



If wo omit thoso lowest of all animals, which Cuviur classed 

 together as nwliato, because their parts were disposed like tho 

 pokes of a whcol, the rest are divided into two great sub- 

 kingdoms. Tho type of the one, called mollunca, is the snail ; 

 and of tho other, named urticuluta, tho honey-bee in the repro 

 eontath . 



It is impossible to say which of these two sub-kingdoms is 

 tho hiuhi'.-t, but they aro very different. That of which tho 

 is tho typo is noted for tho swiftness and agility of the 

 movements of the animals that form it; while thooth'T is equally 

 iviiKirkablofortho 

 sluggishness of 

 the species which 

 compose it. In- 

 deed, the word 

 just used is de- 

 rived from this 

 peculiarity in tho 

 slug. 



These peculiari- 

 ties are, however, 

 but general ones, 

 applying to most, 

 but not all the 

 species of each 

 sub-kingdom ; for 

 each sub-kingdom 

 contains several 

 thousands of dif- 

 ferent kinds of 

 animals. Thus we 

 find some insects 

 more inert than 

 most slugs, and 

 some of tho slug 

 class as active as 

 many insects. 



In accordance 

 with what has 

 been written, the 

 eye of the garden- 

 snail is evidently 

 an organ not at 

 all comparable to 

 eyes we have de- 

 scribed as those of 



the higher classes. Tlu's eye is situated at tho end of tho longer 

 and upper pair of horns, and is only exposed when these are at 

 their longest. Even when so exposed its sense of sight is so 

 obtuse that it seems only conscious of light and darkness, as our 

 skin makes us conscious of heat and cold, and has no knowledge 

 of images. The organ seems little better than a refined organ of 

 touch, for garden-snails will withdraw their eyes far sooner if 

 blown upon, or the hand be placed between them and the light, 

 than when threatened by the fingers. Nevertheless, the eye 

 has a spherical lens, sclerotic, choroid, and retina, but all of 

 very simple structure. The most remarkable circumstance con- 

 nected with this eye is that it can be retracted by drawing 

 it down through the tubular horn, as one might draw the end of 

 the finger of a glove down through tho rest of the finger ; and 

 this is done by a special muscle, which is a slip of tho great 

 muscular band, with which the snail draws in, not only its horns, 

 but its whole head, strongly though slowly. 



The eye is exposed by a successive contraction of the circular 

 muscles which ore round the horn, beginning at the base and 

 ending at the top ; this action has the same effect on the parts of 

 the tube, and finally upon the eye. as driving a coin into the end 

 of on old-fashioned purse by the aid of n ring wliich slides on tho 



outside. The rest of the slugs and snails, which creep on their 



liave eyes somewhat similar, and (similarly situat> : 

 wluli- the garden-snail has four horns, some water-mails have 

 only two, and the eyes are placed on the outside of these, half- 

 way up, while tho whip-like extremities act as feelers, as the 

 short horns of the garden -miai! do. The lower orders of the 

 mollusoa, such as the oyster, etc., hare eyes inferior even to 

 these, though they are sometimes numerous and euriousl* 

 placed; thus, the kind of oyster which occupies the fan-shell, and 

 is called a peoten, has a row of eyes running round the edge of 

 tho two sides of the animal's cloak, which lines the two shells 

 that enclose it. 



Tho highest class of mollnsca hare greater power of motion 

 than any of tho rest, and swim rapidly through tho sea, both 

 backwards and forwards, seizing their prey with long, whip-like 

 arms : and these creatures have large and elaborate eyes, not unliktf 

 thoso of animals, bnt even moro complex in some respects ; foe 

 there is not only a thin retina to receive the light, backed, as the 

 retina always is, by a black membrane, but behind this choroid 

 is another expanded retina, as though this bad some other office 



than to receive im- 

 ir. 



I. VERTICAL SECTION OF THE EYE OF AN INSECT. 



some process ana- 

 logous to the de- 

 velopment of the 

 image in the dark 

 room of the pho- 

 tographer >- 

 fected in this sin- 

 gularly situated 

 organ. The crea- 

 ture whose large 

 eyes have just 

 now been men- 

 tioned has been 

 introduced as a 

 prominent cha- 

 racter in Victor 

 Hugo's " Toilers 

 of the Sea," and 

 the description is 

 probably about as 

 faithful as the de- 

 scription of bri- 

 gands and other 

 horrors described 

 by novelists usu- 

 ally are. 



Turning now to 

 the articulate sub- 



II. THE LENSES AND CONES ENLARGED. in. 



FRONT OF HEAD OF DRAGON-FLY, SHOWING THE POSITION OF THE COMPOUND EYES. IV. FRONT 

 OF HEAD OF WASP, SHOWING THREE SIMPLE AND Two COMPOUND EYES. V. SIDE or CATER- kingdom, we find 

 PILLAR'S HEAD, WITH THREE EYES. in it eyes of tho 



Kef. to Noa. in Figs. I., II. 1, surface lens; 1', layer of paint (iris); 2, cone, vitreous humour; 

 3, special optic nerve ; 4, common pigment ; 5, common retina ; 6, secondary optic nerves ; 



7, main nerve. 



most remarkablo 

 description. They 

 are best explained 

 by the diagram. 



If we examine the head of a wasp or bee, we find on the top 

 of the head, looking towards the sky, three eyes set in a 

 triangle. These eyes are simple, and not unlike the eyes of 

 other creatures ; but besides these, on the side of the head, 

 stretching almost from its crown to the jaws beneath, are two 

 compound eyes, which, under the microscope, are seen to 

 present innumerable six-sided spaces, which look like the ends 

 of the cells of a honeycomb. On dissection, each of these six- 

 sided faces is found to be the outer surface of a double convex 

 lens, behind which is a layer of black paint, which is compara- 

 tively thick at tho edges of the lens, but thin towards the 

 centre, where a hole is left through its middle. This hole is the 

 pupil. Behind the pigment is a cone of transparent matter, 

 whose point is directed inwards, and embracing this point is tho 

 end of a nerve thread. The threads from each eyelet run 

 inwards to a sheet of nervous matter common to the whole eye, 

 and from this sheet other nerve cords, but much fewer in 

 number than tho first, run to the main thick optic nerve. The 

 space between tho nerve cords is filled up with black paint, BO 

 that each can only receive impressions from its end. An insect, 

 therefore, one would think, receives thousands of distinct 

 pictures; but whether it so harmonises them in its commoa 



