86 



MEDUSiE. 



bearing at the apex one or more sometimes spherical, 

 sometimes prismatic, otolitlies. In some cases the 

 organ becomes enclosed in a cup, which in Geryonia 

 closes at the top. 



In another family of the Hydromedusse, the Oceanidas, 

 these organs are absent, and appear to be replaced by 

 certain pigment spots at the base of the tentacles, 

 which, however, from their structure are considered to 

 be rudimentary organs of vision, and will be described 

 in the chapter on eyes. 



Some species have, in addi- 

 tion, other organs, obviously of 

 sense, but the function of which 

 is still far from clear. Fig. 54 

 represents one of these curious 

 sense-organs in Pelagia, after 

 Hertwig. It is in the form of 

 a somewhat bent finger, is 

 situated in a deep fold of the 

 umbrella, contains a branch of 

 the gastro vascular canal, and is 

 filled at the tip with a group of 

 solid, shining, rod-like crystals. 

 The auditory organ in worms 

 and molluscs consists of a 

 closed vesicle, containing one 

 or more otolithes, and lined with nerve-cells, which are, 

 in the higher groups, connected at their base with the 

 auditory nerve, and bear setae at the other end. De 

 Quatrefages was the first who established clearly the 

 existence of auditory organs in worms. 



In the Mollusca, the existence of an organ of hearing 

 in some Gasteropods was justly inferred by Grant from 



Fig. 54.— Sense-organ of Pelagia 

 (after Hertwig). o. Group of 

 crystals ; sk, sense-organ ; sf, 

 fold of the skin ; ga, gastro-vas- 

 cular channel. 



