266 



HISTOLOGY 



some fluid, of unknown function, which is discharged on the surface of 

 the antenna plates. A variety of olfactory tissue in which the perceptory 

 unit consists of an enlarged and multinucleated nerve cell has been 

 described in several grasshoppers. 



The organs of taste in insects have been very satisfactorily located 

 experimentally, but they have not been differentiated structurally from 

 the organ of smell. The palpi are used to taste with, especially on 

 such insects as use them to touch food after it is in the mouth. 



The mollusks also have sensory organs which can better, perhaps, 

 be considered as olfactory rather than gustatory structures. They may 



lu. fp. 



FIG. 232. Central part, and epithelium on one side, of an osphradium plate of Sycotypus 

 canaliculatus , transverse section, b.m., basement membranes on each side; sen.ep., sen- 

 sory epithelium; res.ep., part of respiratory epithelium; lu.ep., portion of lubricating 

 epithelium with one mucous cell, x 700. 



not always be used to test or locate food matter, but are possibly of use 

 in testing the purity of the water or other of its qualities which are of 

 importance to the animal. 



In their form, these cells are remarkably like the earlier stages of 

 the olfactory cells in the vertebrate embryo, having their body drawn 

 out into a short centrifugal process on which the sensory cell organ is 

 developed ; also into a long centripetal process, which acts as the nerve 

 fiber to carry the impulse to the central ganglion. In regard to their 

 tissue organization, we may find what appear to be two kinds of organs : 

 one which is found as part of a modified gill, the osphradium, and an- 

 other which is developed from the inner, mantle epithelium, in several 

 places, in the cephalopod mollusks. 



