SENSE ORGANS. As soon as sense organs ap pear the nervous elements of animals consist of three parts: (I) the so-called terminal, end, or sense organ; (2) the ganglia; and (3) the nerves. End organs appear first clearly defined in the Ccelenterata, although the pigment spots of Euglena are in a particularly sensitive region in that infusorian. The sense organs of the Coe lenterates are most developed in the and consist of the tentaculocysts with calcareous lith ites, pigment spots or ocelli, and sensory pits known as olfactory pits. Pigment cells or 'eye spots' are found in many of the flatworms. some of the polyclads possessing several hundred. These eyes, in many forms, are grouped in pairs at the anterior end of the body or are scattered along the anterior margin, or even along the entire margin of the body. In many forms the eyes are simple pigment spots. In others a re fractive body occurs. In trichuls and polyclads the eye consists of a pigmented apparatus with retinal or light-perceiving cells within a cup shaped organ. These cells connect with the optic nerve. Auditory organs also occur in the flatworms, each consisting of a fluid-filled sphere lying close to the brain in which is an otolith. The is everywhere very sensitive, due to tactile hairs, especially on the tentacles. The proboscis of certain forms contains an invardna tion which is capable of extrusion, and is be lieved to be a touching organ. The suckers of trematodes, too, are especially sensitive. 'there are ciliated pits in the epithelium of triclads and other forms to which an olfactory function lies been ascribed.
Sense organs occur over the whole integument of mollusks. These cells, when in exposed posi tions. may have a tactile function. The pallial sensory organs and the ospharadium, which occur near the gills, are probably olfactory organs to test the condition of the water. The optic tentacles of pulmonates and the dorsal ten tacles of opisthobranchs are stated by sonic au thorities to be olfactory organs. Ciliated ridges in the mantle furrows of certain ehilons are supposed to have an olfactory function. Auditory organs are absent in the Amphineura alone among mollusks. The auditory organ consists of a closed, flnid-filled vesicle, or 'otoeyst,' in which a varying number of otoliths are inclosed.
Sensory cells occur on the wall of the otocyst, which bear sensory hairs. The otoeysts of cephalopods have been shown to have a statieal function also. The simplest visual organs of mollusks are optic pits. The retina lies at the base of the pit, and along this base the optic nerve spreads out. The retina consists of cells with and without pigment. External to the retina is a gelatinous cuticle or even a lens. By the approach and fusion of the edges of the pit the vesicular eye is formed. '1 hat part of the epithelium which is free from pigment consti tutes the cornea. This type of eye occurs on the tentacles of gastropods. By a further develop ment the simple cup-like eye is differentiated into the complex. highly developed organ of cephalo pods. The mantle eyes of the scallop resemble vertebrate eyes in that the visual rods are turned away from the light, that is to say, they are directed toward the body.
The sense organs of echinoderms comprise the auditory organs and eyes. The auditory organs are of two types. The first type is represented in holothurians, and consists of an otocvst which is lined with ciliated epithelium, container and otolith, and filled with fluid. In echinnids is found the second type of auditory or orienting organs, winch are believed to be transformed spines. Organs of this type are known as `splifericlia.' and are developed only on the oral side; they are spherieal,stalked bodies and may be inclosed in a pit of the test. The organs of special sense in the annelids are the eyes, ten tacles, cirri. and otoeysts. The structure of the eye, when that organ is present, is very simple. The eyes are confined to the prostomium, hut there are eye-like organs on many or all the body segments or on the branehial filaments of many species. Eyes are lacking in nearly all Oligoeleta and sonic of the Polyehreta. The eye is usually a capsule with a layer of elongated cells on the side toward the brain. A thickening of the cuticle may form a lens. Otocysts with otoliths are not common. Tactile cells occur generally all over the body and especially on the prostomium.