INTRODUCTION. 



dilatory centres. The mouth is on the lower anterior surface of the body, and is sur- 

 rounded by the " mouth-parts " (the appendages of the adjacent segments, variously 

 modified for the purposes of eating). These are sometimes adapted for crushing and 

 biting, at others for piercing and sucking. Usually these mouth-parts are capable of 

 motion, but they move in a horizontal plane, from side to side, and not vertically, as do 

 the jaws of vertebrates. From the mouth the oesophagus passes upward and backward 

 through the nervous system, as has been described, and terminates in the Crustacea at 

 the stomach, in the insects at the crop or ingluvies. The rest of the course of the 

 alimentary canal presents so many variations that it is best to resume the subject 

 in connection with the different groups, and to close our account of it here with 

 the statement that in many forms the canal is a simple tube with no well-defined 

 divisions. 



In the Arthropods two types of eyes are found, simple and compound, both some- 

 times occurring together in the same animal. 

 These eyes have recently been made the sub- 

 ject of very exhaustive studies by Grenacher 

 and others, but their accounts are too long for 

 detailed insei'tion here. The simple eyes consist 

 of a thickening of the outer integument, forming 

 at once a refractive lens and an organ of defense. 

 Beneath this lens the cells of the hypodermis be- 

 come elongated. In the outer cells pigment is 

 deposited, but the inner are transparent, and the 

 lower ones, which are in connection with filaments 

 of the optic nerve, form a retina. Various modi- 

 fications of this structure are found, all, however, 

 being reduceable to this type. In most cases the 

 anterior portion of the retinal cells become elon- 

 gated into a " rod," while the anterior transparent cells frequently break up into a cor- 

 responding number of highly refractive bodies, known as crystalline cones. Each of 

 these cones has its base placed against the corneal lens, while its tip is connected with 

 a rod, and thus with the optic-nerve. 



The compound eyes differ from the simple ones by having a large number of cor- 

 neal lenses, each conveying the light to a single rod and cone. In some (Fig. 3) the 

 crystalline cones are well developed, in others {Fig. 2) they are represented by cells 

 but little modified. Concerning the physiological action of the eyes of Arthropods, 

 there have been several theories, but the one which at present is most in vogue is the 

 " mosaic theory," which supposes that each retinal cell perceives but a portion of the 

 picture, and that by the action of the brain the various parts are put together so that 

 the whole is seen, as a veritable optical mosaic. There are certain difficulties connected 

 with this theory, but our space forbids their discussion. 



As a rule the eyes are confined to a Avell-defined region, that of the head, there 

 being, however, one conspicuous exception, the crustacean JZuphausia, where on the 

 thorax and abdomen occur organs which are interpreted as having visual functions. 

 In many of the Crustacea simple eyes are found in the young, but all traces of them 

 disappear in the adult. 



The auditory organs are far less constant in their position, as can be seen from a few 

 examples. In the Decapod Crustacea they occur on the basal-joint of the inner an- 



FIG. 1. Section of simple eye of Dytlscus 

 larva, greatly enlarged. I. Lens. c. Crystalline 

 body, not yet broken up into cones, r. Rods. 

 rt. Retina, n. Optic nerve. 



