XXX THE ANIMAL KINGDOM. 



The tactile sense, or sense of touch is common to all animals ; this is the most fun- 

 damental sense, of which the other senses are without doubt differentiations. In the 

 lower Protozoa, such as the Amoeba, the sense of touch which they appear to possess 

 may be due to the inherent irritability and contractility of the protoplasm of which 

 their bodies are formed. 



In the Infusoria, without doubt, the cilia and the flagella with which these animals 

 are provided are not only organs of locomotion but also of touch. It is probable that 

 none of the many-celled animals are without the sense of touch unless some of the 

 sponges, and the root-barnacles (SaccuUna) may be, by reason of their lack of a nervous 

 system and otherwise degenerate structure, destitute of any sense whatever. 



The most important of the sense organs are undoubtedly eyes, as they are the 

 most commonly met with. The transparent spot in the front of the body of 

 Euglena viridis, a protophyte, may possibly be the simplest of all sense organs ; if so, 

 it anticipates the eye of animals. The simplest forms of eyes are perhaps those of the 

 sea-anemone, in which there are, besides pigment cells forming a colored mass, refrac- 

 tive bodies which may break up the rays of light impinging on the pigment spot, so 

 that these creatures may be able to distinguish light from darkness. The next s*ep in 

 advance is where a pigment mass covers a series of refractive cells called crystalline 

 rods or crystalline cones, which are situated at the end of a nerve proceeding from the 

 brain. Such simple eyes as these, often called eye-spots, may be observed in the flat 

 worms, and they form the temporary eyes of many larval worms, echinoderms, and mol- 

 luscs. In some nemertean worms, such as certain species of Polia and Nemertes, true 

 eyes appear, but in the ringed worm, Neophanta celox, Greef describes a remarkably 

 perfect eye, consisting of a projecting spherical lens, covered by the skin, behind which 

 is a vitreous body, a layer of pigment separating a layer of rods from the external part 

 of the retina, outside of which is the expansion of the optic nerve. Eyes are also sit- 

 uated on the end of the body in some worms, and in a worm called Polyophthalmus, 

 each segment of the body bears a pair of eyes. 



The eyes of molluscs are, as a rule, highly organized, until in the cuttle-fish the 

 eyes become nearly as highly developed as in fishes, but still the eye of the cuttle-fish 

 is not homologous with that of vertebrates, since in the former the crystalline rods are 

 turned towards the opening of the eye, while in vertebrates they are turned away from 

 the opening of the eye, so that, as Huxley as well as Gegenbaur show, the homology 

 between the eye of the cephalopods and of the vertebrates is not exact. 



While, as we have seen, the eyes of the worms and the molluscs are situated arbi- 

 trarily, by no means invariably placed in the head, in the crustaceans the eyes assume 

 in general a definite position in the head, except in a schizopod crustacean (JEwphausid), 

 whei'e there are eye-like organs on the thorax and abdomen. In insects there are both 

 simple and compound eyes occupying definitely the upper and front part of the head. 



The eyes of the lanoelet are not homologous with those of the higher vertebrates, 

 being only minute pigment spots comparable with those of the worms. In the skulled 

 vertebrates the eyes are of a definite number, and in all the types occupy a definite 

 position in the head. 



The simplest kind of auditory organ is to be found in jelly-fishes, where an organ 

 of hearing first occurs. In these animals, situated on the edge of the disc, are minute 

 vesicles containing one or more concretionary bodies or crystals. Reasoning by ex- 

 clusion, these are supposed to represent the ear-vesicles or otocysts of worms and 

 molluscs ; and the concretions or crystals, the otoliths of the same kind of animals. 



