116 



COELENTERATA. 



ectoderm cells of the pit or vesicle contain otoliths, while others bear sensory 

 hairs (Fig. 104). In the other type, found in the Trachomcdusac and Narco- 

 medusac, the auditory organs have the form of short reduced tentacles of the edge 

 of the disc, containing a solid endodermal axis and surrounded at their base by cells 

 bearing long auditory sense hairs. The otoliths are contained in the endodcrm 

 cells of the tentacles. These sense tentacles may project freely on the surface 

 (Acginidac, Fig. 96), or they may be sunk in pits, or the pits may be closed 

 and they may lie in vesicles embedded in the jelly of the umbrella (Fig. 102), yi 

 which case they may be fitly termed tentaculocysts. In both types the hair- 

 bearing sensory cells are prolonged into fine fibres which join the nerve rings 

 (Fi 



g. 105). 



FIG. 103. Section through the olfactory pit (fi), the sense tentacle (marginal body), and its 

 nerve centre of Am-i'lin nui-lta. L lobe of umbrella covering the sense tentacle ; P eye-spot; 

 Ot otoliths in the endoderm of the sense tentacle ; Z endodenn cells after solution of the 

 otoliths ; En endoderm ; EC ectoderm with the underlying tissue of the nerve centre F. 



The eye-spots of the Craspcdota (ocellate medusae or Anthomedusae) consist of 

 a pigmented patch of ectoderm cells just dorsal to the insertion of the velum ; 

 a lens-like cuticular thickening over them is sometimes developed (Lizzia). 

 The ectoderm patches contain two kinds of cells, namely peripheral cells with 

 pigment surrounding some colourless cells, the bases of which are prolonged into 

 nerve fibres. 



The marginal bodies of the Acraspcda or the rhopalia are absent in the 

 Tcsseridae, and in most Lucernaridae. When they are present they have the form 

 of short marginal tentacles and are distinguished from those of the Cmspedota 

 by the fact that they contain a hollow prolongation of the gastrovascular system, 

 and are covered on the dorsal side by a hood-like prolongation of the umbrella 



