Polyps, Jelly-fishes and Sponges. 



a diameter of six inches or more ; its colour is usually pale 

 blue or pale green. This JellAr-fish is quite harmless, as its 

 stmging-cells are not sufficienth' powerful to pierce the human 

 skins. The allied Chrysaora isosceles (Fig. 88), capable of 

 inflicting a most painful sting, can be recognised by its larger 

 oral lips and by the twenty-four stout tentacles which depend 

 from the bell margin and replace the fringe of small tentacles 

 which occurs in Aurelia. The yellow and blue species of 

 Cyanma, C. capillata and C. lamarcki are also forms with 

 powerful stinging organs ; in these the bell margin is deeply 

 lobed and the long, slender tentacles are grouped in eight 

 tufts on the ventral surface. 



Rhizostoma pulmo (Fig. 89) is a common Jelly-fish in the 

 English Channel ; it is peculiar in having the large oral lips 

 fused over the mouth in such a way that this opening is almost 

 obliterated, being reduced to a number of minute apertures 

 arranged along the lines of suture, and capable of taking in only 

 the tiniest food-particles. This is a large, handsome form, the 

 deep bell of which may attain a diameter of two feet, and is 

 not provided with marginal tentacles. The colour is pale green 

 or blue, with a deep blue or purple edge. Like the common 

 Aurelia, this Rhizostoma may be handled with impunit}?. 



As in this chapter we have continually referred to the 

 stinging or urticating powers of polyps and jelly-fishes, a short 

 explanation as to how this is produced may be of interest. 

 In the tissues of all these forms very highty modified cells, 

 known as stinging-cells, are to be found, either scattered over 

 the surface of the body or tentacles, or, more commonly, grouped 

 in clusters or batteries. Each cell contains an oval capsule 

 filled with a poisonous fluid, as well as a coiled thread provided 

 with small barbs ; on appropriate stimulation these threads are 

 shot out \vith great violence, and penetrate the bodies of any 

 organisms with which they come into contact. When once 

 discharged, the stinging-cell cannot be used again, and is cast 

 off, being replaced by a new one, ready to take up its position 

 at the surface of the body. While the sting of the average 

 polyp or jelly-fish is not ver}' formidable, it is sufficiently power- 

 ful to render these creatures distasteful to the majority of 

 predaceous animals. They are also singularly free from parasites, 

 although many small animals associate with them in a less 

 intimate manner ; thus, several large jelly-fishes are constantly 

 accompanied bj' shoals of shrimps or small fishes, which, when 

 disturbed, seek shelter in the area protected by the long stinging 

 tentacles. There are no jelly-fi,shes of real economic value, 



