Sensory Discrimination: The Chemical Sense 87 



§ 21. The Chemical Sense in Crustacea 



The highest invertebrate ammals belong to the phylum 

 of the Arthropoda, like the annelid worms in their segmented 

 structure, but more highly organized in many respects. 

 The body of a typical arthropod consists of a series of seg- 

 ments, one behind another, each segment with a pair of 

 appendages. The higher an arthropod stands in the 

 scale, the more modification and differentiation of func- 

 tion there is in the segments and appendages ; the former 

 often become consolidated, and the latter become modified 

 for swimming, walking, or sensory purposes. The lowest 

 grand division of the Arthropoda is that of the Crustacea. 



As the animals of this group are covered with a hard out- 

 side shell, sensitiveness to touch and chemical stimulation 

 is ordinarily referred to certain hairs scattered over the 

 body, and to the modified appendages of the anterior seg- 

 ments which we commonljr know as "feelers," the large 

 and small antennas. That mechanical contact stimuli 

 in certain Crustacea give rise to specialized reactions 

 is evidenced by observations on the hermit crab. This 

 animal, as is well known, has acquired the instinct of tak- 

 ing up its abode in empty shells, most commonly those of 

 some gasteropod moUusk. When wandering about in 

 search of a dwelling, the crab's reactions to the objects it 

 meets show adaptation to the character of the stimulus, for 

 it will not investigate a glass tube or ball ; the smooth sur- 

 face seems not to be the adequate stinlulus for beginning the 

 movements involved in exploring and entering a shell (194). 



The responses of Crustacea to food stimulation vary, as 

 might be expected, with different genera and species. 

 Nagel finds the r61e of the food sense in aquatic Crustacea 

 very insignificant ; they occasionally show antennal move- 



