II 



CNID ARIA SENSORY ORGANS 



95 



nr, 



II. Tentaeuloeysts or 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-meduscK, Fig. 72, B, C), or 

 the peripheral distal end cells of the 

 hollow tentacle-canal (Acraspedt.i, 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 Trachomedusce 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- 

 eulocysts 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- FlG . 72> _^ Auditory vesicles of 



pound sensory organs, of which the .Slquorea Forskalea. B, Tentaculocysts 



auditory organ is the principal factor. f cunina lativentris. c, Tentaculocysts 



rp, , . r of Rhopalonema velatum. A and B, Trans- 



Inese are, at the same time, meta- verse sections of the margin of the disc ; c, 



morphosed tentacles into which a section of the margin of the disc, after 



hollow process of the gastro- canal * 6rtWig ' ?' Eptoderm; er, endoderm of 



the circumferential canal ; nri, upper ; nr.->, 

 SVStem penetrates. The endodermal lower nerve ring ; r, circumferential canal"; 



cells at the peripheral blind end of sm > supporting membrane; , otoiith; hh, 



it. n , T,I auilitory hairs ; //.:, auditory cells : hk. tenta- 



tnese processes produce an otoiith or cu i ocys t s; w, auditory vesicles- g jetty 



a mass of Otoliths. The OUter epi- m, muscle lamellre ; en, endoderm cells of 



thelium of the auditory body carries the tentac uicysts. in A and E the velum 



,v T, i T i is bent centrifugally outwards. 



the auditory hairs. In it, also, on 



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



