392 ZOOLOGY. 



endoderm are in close contact, except in the course of the 

 radial canals and circular canal, where the two layers 

 separate to form these canals. Thus the umbrella is to be 

 regarded as due to the approximation of the upper and 

 lower walls of a flattened sac, and the mesogloea is every- 

 where between endoderm and ectoderm, as the general 

 structure of coelenterates requires. The double layer of 

 endoderm extending between the radial canals is called the 

 endoderm lamella, and if this were not present the meso- 

 gloea of the umbrella would be between two layers of 

 ectoderm, as it never is in coelenterates. 



RADIAL 

 CANAL 



Fig. 200. DIAGRAM OF A VKRTICAL SECTION OF A MEDUSA. 



The endoderm is represented by a black line, the ectoderm by a band of transverse 

 lines, the mesogloea by dots. The section really consists of two separate halves, 

 that on the left passing through a radial canal, that on the right through the endo- 

 derm lamella between the canals. 



6. Tentacles and Sense Organs. Around the margin 

 of the umbrella are numerous tentacles, whose number 

 increases with the age of the medusa. The position of 

 these tentacles and other organs is fixed by reference to 

 radii of the circular umbrella. The four radial canals fix 

 the position of four principal radii, called per-radii, and 

 at the ends of these are four per-radial tentacles. Inter- 

 mediate between the per-radii are four interradii with 

 interradial tentacles. Bisecting the angles between these 

 are eight adradii with adradial tentacles. The tentacles 

 are somewhat swollen at their bases and on the lower side 

 in all of them there is a little spot where the ectoderm 

 cells are piginented. These pigment spots are probably 

 sensitive to the stimulation of light, but have not the 



