MORPHOLOGY OF ANIMALS 



various kinds of stimuli, such as mechanical, thermal, elec- 

 trical, and chemical, and are therefore largely undifferen- 

 tiated, though probably chiefly subserving the sense of touch. 

 These integumentary sense organs are found in almost every 

 group of animals. Among the vertebrates they are pres- 

 ent in primitive form over the general body surface; in the 

 fishes and amphibia they are aggregated into buds, forming 

 the lateral line organs^ while among the vertebrates which do 

 not dwell in water, deeper-lying organs, of modified type, are 

 found (^tactile cells^ corpuscles^ and hulbs^ Fig. 7, np). In ad- 

 dition to these organs of general sensation, higher Metazoa 

 generally possess specific sense organs namely, those differen- 

 tiated for the reception of particular kinds of stimuli. 

 These are organs of ( 1 ) smell and taste, (2) equilibrium and 

 hearing, (3) vision. 



(1) Organs of smell and taste are organs for receiving 

 chemical stimuli; they are present in all vertebrates and in 

 many invertebrates. Their structure is extremely simple, 

 being but slightly modified from the type of the primitive 

 organs described above. In fact the olfactory sense cells of 

 vertebrates are merely scattered sensory cells, while the 

 organs of taste (taste buds. Fig. 25, A) are simply aggre- 

 gations of such cells. Throughout the Metazoa the organs 

 of taste and smell are generally located in ciliated pits or 

 depressions of the integument either on the head or at least 

 near the mouth or respiratory organs. In these positions they 

 serve in the one case to test food and in the other the quality 

 of the medium used in respiration. Among fishes the olfac- 

 tory organs are located in pits on the front of the head; in all 

 air-breathing vertebrates these pits open posteriorly into the 

 mouth-cavity or pharynx, and thus form the anterior part of 



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