82 The Animal Mind 



was placed on the back it remained unnoticed (365, pp. 321- 

 322). Preyer reported great individual differences in the 

 responses of starfish to food stimulation ; while certain speci- 

 mens were unmoved by the neighborhood of food, an indi- 

 vidual of another species came from more than six inches 

 away and fell upon it (350). Whether the unlikeness of 

 behavior was due to the species difference or to a difference 

 in the degree of hunger, does not appear. 



23. The Chemical Sense in Crustacea 



The highest invertebrate animals 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 segments, 

 one behind another, each segment with a pair of appendages. 

 The higher an arthropod stands in the scale, the more modi- 

 fication and differentiation of function there is in the seg- 

 ments and appendages; the former often become consoli- 

 dated, 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 segments which 

 we commonly know as "feelers," the large and small antenna?. 

 That mechanical contact stimuli in certain Crustacea give rise 

 -JLto specialized reactions is evidenced by observations on the 

 hermit crab. This animal, as is well known, has acquired 

 the instinct of taking up its abode in empty shells, most com- 

 monly those of some gasteropod mollusk. When wandering 

 about in search of a dwelling, the crab's reactions to the ob- 

 jects it meets show adaptation to the character of the stimulus, 



