CNIDARIA SENSORY ORGANS 



95 



ec 



nr, 



II. Tentaeuloeysts OP auditory tentacles are, among the Medusce, 

 the most widely spread organs of 

 hearing. They are short transformed 

 tentacles in which the solid endo- 

 dermal axis in the Craspedota (Tracho- 

 and Narco-medusce, Fig. 72, B, C\ or 

 the peripheral distal end cells of the 

 hollow tentacle-canal (Acraspeda, Fig. 

 73), form one or more otoliths, which 

 are here, therefore, endodermal. The 

 ectoderm of the tentaculocysts of the 

 Craspedota and often also a sensory 

 cushion round the base of the tenta- 

 culocyst carry long stiff auditory 

 hairs. In many TracliomeduscK the 

 sensory cushion rises round the base 

 of the tentaculocyst into a circular 

 wall (Fig. 72, C\ which may even 

 completely close over the tentaculo- 

 cyst. Thus closed vesicular auditory 

 organs again arise, but these have 

 quite a different origin and morpho- 

 logical signification from the true 

 marginal vesicles of the Leptomedusce. 



Between the inner wall of the 

 auditory vesicle and the tentaculocyst 

 which it encloses, the auditory hairs 

 are stretched like chords. The tenta- 

 euloeysts always receive their nerves 

 from the exumbrellar nerve ring. In 

 the simplest cases we find 4 interradial 

 tentaculocysts, but their number is 

 often considerably greater. 



The sensory bodies or rhopalia 

 of the Aeraspeda (Fig. 73) are com- 

 pound sensory organs, of which the 

 auditory organ is the principal factor. 

 These are, at the same time, meta- 

 morphosed tentacles into which a 

 hollow process of the gastro- canal 

 system penetrates. The endodermal 

 cells at the peripheral blind end of 

 these processes produce an otolith or 

 a mass of otoliths. The outer epi- 

 thelium of the auditory body carries 

 the auditory hairs. In it, also, on 

 one side lie the eye or eyes ; close to it also lie the olfactory pits. The 



FIG. 72. A. Auditory vesicles of 

 -ffiquorea Forskalea. B, Tentaculocysts 

 of Cunina lativentris. C, Tentaculocysts 

 of Rhopalonema velatum. A and B, Trans- 

 verse sections of the margin of the disc ; C, 

 section of the margin of the disc, after 

 Hertwig. ec, Ectoderm ; er, endoderm of 

 the circumferential canal ; nri, upper ; nr%, 

 lower nerve ring ; r, circumferential canal ; 

 sm, supporting membrane; o, otolith; hh, 

 auditory hairs ; hg, auditory cells ; Me, tenta- 

 culocysts ; bl, auditory vesicles; g, jelly; 

 m, muscle lamellae ; en, endoderm cells of 

 the tentaculocysts. In A and B the velum 

 is bent centrifugally outwards. 



