11. Organs. 5. Sense Onjan.'i. 



19 



tlie tip, and continued at the other end into a nerve fibre, which 

 passes to the front part of the brain. Amongst Insects, which have 

 been proved by many experiments to possess a very acute sense of 

 smell, the olfactory organs are situated upon the antennae. These are 

 provided with delicate hairs, which, like arthropod hairs in general, 

 are evaginations of the cuticle ; they are very thin walled, generally 

 seated in small depressions, and into each, a thread-like process 

 extends from one or more underlying sense cells (Fig. 18). 



Whilst these insectan structiu'es are 

 probably true olfactory organs, there are 

 mimerous contrivances to which, even in 

 this book, the same name is applied, but 

 ■with doubtful right. This is the case with 

 aU the " olfactory organs " of aquatic 

 animals, which seldom or never come in 

 contact with air. The olfactory pits of 

 Fish, which, from their position, &c., are 

 homologous* with the olfactoiy organs of 

 higher Vertebrates, can, for instance, hardly 

 be really olfactory. This is also tnae of 

 the delicate " olfactory hairs " present on 

 the antennules of many Crustacea ; like the 

 above-mentioned structures in Pish they are 

 undoubtedly sense organs, but their special 

 function is unknown: they are, perhaps, 

 organs of taste. 



Gustatory Organs are affected 

 only by substances which occur in 



a liquid form. In the Vertebrata they are represented by the 

 so-called taste-buds of the tongue and the walls of the mouth, 

 specially modified tracts of the buccal epithelium, consisting of 

 groups of cells, amongst which are long thin cells with a delicate 

 point projecting from the free end ; these are, probably, the true 

 organs of taste. The buds are supplied with nerve fibres which 

 branch between the cells. In Fish, taste-buds may occur on the 

 external surface of the body ; similar structures are found also in 

 many Chsetopods and Molluscs {e.g., Gastropods), not only in the 

 buccal cavity but also externally upon the anterior end of the body. 

 Of a difierent type are the organs which appear to bring about 

 sensations of taste in Insects ; they are short hairs placed on the 

 underlip, jaws, etc., and are in all essential respects like the olfactory 

 hairs just mentioned. 



Tet other structures are included under the simple sense organs, but of their 

 special function nothing is definitely known. Such are the lateral line organs of 

 Ksh (See Pisces) and Amphibia, and the similar stniotui'es in Annelids and 

 Molluscs. 



Auditory Organs always occur as vesicles filled with fluid. 

 The vesicles (otocysts) are formed by epidermic invaginations, and 



Fig. 18. Section throngli a small 

 piece of the antenna of an Insect, 

 diagrammatic ; c cuticle ; g pit, with 

 r the " olfactory hair," arising from the 

 thin part of the cuticle ; ep epidermis, 

 sz sense-cell. — Orig. 



■ See conclusions under Section V. 



c 2 



