SENSE ORGANS 



157 



guish food from other objects in such a way as to indicate a 

 chemical sense. No specialized chemical sense organs have 

 been distinguished. 



368. The earthworm is chemically sensitive over the entire 

 surface of the body, but at the anterior end this sense is best 

 developed. The function seems to be located in sensory cells, 

 which occur in clusters, the clusters being distributed over the 

 surface of the body in numbers which correspond approxi- 

 mately to the sensitiveness of the region. 



369. The antennules of the crayfish are the seat of chem- 

 ical sense. The sensory cells concerned arc not specially 

 modified, but their accessory terminal end-organs are peculiar 

 club-shaped hairs, which are covered by an extremely thin 

 cuticula. In insects, there is a differentiation of the chemical 

 sense into an olfactory and a gustatory sense. The former is 

 located on the antennae, and the sense organ consists of a flask- 

 shaped cluster of sense cells which are exposed at the surface 

 at the bottom of a pit in the cuticula. The organs of taste 

 are similar, but are found on surfaces bounding the mouth 

 cavity or on the mouth appendages. 



370. In arthropods we first observe a decided hmitation of 

 the chemical sense organs to the region of the mouth and in the 

 air breathers, the first differentiation of the senses of taste and 

 smell. In some fishes, organs of taste are found on the surface 



Fig. 77. — Sense organs. A-E, General sense organs; F-II, organs of taste; 

 /, olfactory organ; J-0, eyes. A, General sense organs of nereis; B, general 

 sense organs of Arthropods; C, free nerve terminations in the epidermis of 

 Vertebrates; D, sensory corpuscles in the dermis of Vertebrates; E, same, en- 

 larged; F, diagram of human tongue showing distribution of fungiform (i) and 

 circumvallate papillae; G, section through a circumvallate pajjilia showing 

 position of the taste buds; //, section of a taste bud, showing two sensory cells, 

 one supporting cell (c) and a nerve fibre (g); I, section of the olfactory epithelium; 

 /, outline of a small "gliding worm" with two simple eyes; A', anterior md of 

 another "gliding worm" with a number of simple eyes arranged along the edge of 

 the body; L, eye of a ''gliding worm" consisting of a single cell with a sensory brush 

 partly surrounded by a pigment cell; M, eye of a snail (Patella); .V, eye of 

 nereis; O, part of N, on a larger scale, r. Supporting cell; (';/, cuticula; D, 

 dermis; Ep., epidermis; g, fibre from a ganglion cell; //, sensory hair, or bristle; 

 N.C., nerve cell; N.F., nerve fibres; O. N., optic nerve; P, pigment; S, sense cells. 



