ORGANS OP THE SENSES. 



297 



organs of sense may compensate for the deficiency, and 

 enable them amply to provide for all their wants. The 

 naked surface and long cilia, almost developed into ten- 

 tacula, of most polygastric animalcules, the long sensitive 

 tentacula of most zoophytes and acalepha, and the fleshy 

 tubular feet of higher radiated animals, are the parts most 

 adapted to receive impressions of this kind, although we 

 perceive no distinct organs in these animals appropriated 

 solely to the sense of touch. These sensitive organs con- 

 tinue soft in the helmmthoid articulata, but become con- 

 solidated and jointed in the entomoid classes where they 

 constitute the various forms of palpi and antennae. There is 

 one pair of these antennae in the myriapods as in most of 

 the annelides, and the same number is seen in the insects, 

 but they are deficient in the arachnida, and two pairs are 

 found in the Crustacea where they are generally more ex- 

 tended than in insects. In the molluscous classes they 

 again assume the soft, sensitive and fleshy condition of 

 tentacula, destitute of articulations, as we observe around the 

 orifices of the respiratory sac in unicated animals, and around 

 these orifices and the margins of the mantle in conchifera. 

 In the gasteropods there are commonly two of these ten- 

 tacula, as seen in the cyprcea erosa, (Fio. 106. c. c. 9 ) they 



extend from the sides of the 



FIG. 106. 





neck or the mouth, (106. b.,) 

 supporting the eyes (106. 

 d. d. } ) near their base, and 

 are often surmounted by the 

 ciliated syphon (106. a.) for 

 the passage of water to the 

 respiratory organs. The 

 muscular foot (106. e.) pos- 

 sesses exquisite sensibility ; 

 there is frequently a second 

 pair of tentacula at the sides 

 of the mouth, and numerous 

 fleshy extensions, simple or 

 ramified (106. /.), are often 

 seen prolonged from the 

 sides or the general surface, 

 of the mantle. Similar fleshy 

 cephalic tentacula are seen 



