AIJRELIA 



103 



The sense-organs (Fig. 42, s.i) are highly characteristic : they are 

 eight in number, situated round the margin of the umbrella per-radial 

 and inter-radial in position, and are really tentacles which have become 

 much shortened and modified to form sense-organs. Each is traversed 

 by a tubular cavity continuous with the ring-canal and lined with endo- 

 derm. The somewhat swollen end of the tentacle is occupied by a 

 solid mass of endoderm cells continuous with the lining of the tube 

 already mentioned and the cells forming this secrete numerous masses 

 of calcium carbonate so that the end of the tentacle is heavily 

 weighted. 



In Aurelia the sensory tentacle projects freely, although it is sheltered 

 by a hood-like arrangement, but it is interesting to note that in some other 

 medusae similar sense-tentacles come to be surrounded by a wall-like 



Fig. 43. 



Illustrating the enclosure of a sensory tentacle within an otocyst in a Medusa — Rhopalonema. 

 (After O. & R. Hertwig.) In the younger stages {A and B) the tentacle projects freely. In the 

 later stage shown in C it is in process of being enclosed. In still later stages the opening of the otocyst 

 becomes completely obliterated. 



upgrowth which eventually arches over and causes them to be completely 

 enclosed in an otocyst (Fig. 43). 



As in the Hydromedusae so also in the Acalephae a polyp phase 

 occurs in the life-history, but it is small and inconspicuous as compared 

 with the medusoid phase. The eggs are fertilized within the stomach 

 by microgametes which have come in from the exterior and the zygotes 

 so formed pass out through the mouth opening and along the groove 

 extending from it along the arm-like prolongations of the angles of the 

 mouth. Here they become lodged in pocket-like outgrowths of the 

 groove in which the^ proceed with their development. The zygote under- 

 goes a process of segmentation which results in a spherical blastula. One 

 wall of this becomes tucked within the other very much as one hemisphere 

 of a child's indiarubber ball may be pushed within the other (Fig. 44, A). 

 In this way is reached a very important stage of development known as 



