Sensory Organs 41 



which terminate in the muscular structures of 

 the manubrium and other parts of the animal. 



In connection with the nervous system of 

 Medusoidswe find tactile sensory organs, similar 

 to those which exist in Hydra, spread over their 

 tentacles and over certain regions of their 

 bodies. In 

 addition to 

 these sensory 

 organs some of 

 the Medusae 

 present definite 

 structures de- 

 rived from the 

 ectoderm or 

 living s u b- 

 stance of the 

 outer layer of 

 cells, which are 

 described as statocysts and ocelli ; these organs 

 are for the most part located on the margin of 

 the bell, at or near the base of the tentacles. 



The statocysts consist of minute masses of 

 organic and calcareous matter, supported in a 

 cavity containing sensory fibres, which com- 



FIG. 5. Statocyst. d 1 , superficial layer 

 of ectoderm ; d 2 , deeper layer ; h, specia- 

 lised cells of ectoderm ; hh, supporting 

 filaments ; np, nervous structures ; npr, 

 upper nerve ring ; r, endoderm ring of 

 circular canal. The calcareous body 

 and cavity of eense-organ seen above. 

 (Hertwig.) 



