MORPHOLOGY OF ANIMALS 



26, A). In other jellyiishes the sensory cells may entirely 

 enclose the tentacle, thus forming a vesicle or statocyst. The 

 organs of equilibration and hearing of most vertebrates, 

 as well as of most invertebrates, can be traced back to this 

 simple type. The sensory cells forming the walls of the stato- 

 cyst are similar to tactile cells, — that is, they bear processes 

 projecting into the cavity of the statocyst (Fig. 26, B). 

 By the movements of the statolith, usually a calcareous con- 

 cretion, these cells are stimulated and the impulses thus 

 generated are conveyed away by the nerve fibre. Statocysts of 

 this type are possessed by mollusks, certain annelids, tur- 

 bellarians, and brachiopods. 



Arthropod Statotcysts and Ears. In the case of arthro- 

 pods organs of a different type are usually found, owing to 

 the fact that the body is here covered with chitin and that 

 the fine protoplasmic processes or cilia are absent. Among the 

 higher crustaceans the auditory organ usually consists of a 

 cavity in the basal joint of the first antenna (Fig. 25, B), 

 which is open to the exterior and which contains water and 

 grains of sand; the wall of the cavity bears chitinous pro- 

 cesses or auditory hairs which have a nervous connection at 

 their base; these hairs are stimulated by the movement of 

 water and sand within the auditory sac. Many insects have 

 a true tone-perceiving organ, the chordotonal organ; in 

 principle this consists of a few elongated cells, the chord, 

 one end of which is attached directly to the integument, 

 while a ligament runs from the other end to an opposite point 

 of the integument; when this apparatus is thrown into vibra- 

 tion, impulses are conveyed to the nerve cells attached to 

 some portion of the chord. In other insects (Orthoptera) a 



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