NATURAL SCIENCES OF PHILADELPHIA. 



763 



1902.] 



Irregularities are common in the appearance of the tentacles of the 

 polyp, as in the adult. Il frequently happens that only one of the 

 second pair ever makes its appearance. Or one may be slow in 

 arising, and always remain smaller than the other. On the other 

 hand, an abnormally large number are frequently developed, indi- 

 viduals wilh five or six being not uncommonly found (Fl. XXXII, 

 fig. 13). 



B. Form of Ci^^lenteron.—The appearance of the tentacles is 

 accompanied by alterations in the form of the coelenteric cavity. 

 The rapid growth of the cells al the points where the tentacles arise 

 and the outpushiug of the tissue in the process seem to affect the 

 contour of the body-wall 

 over a considerable area, so 

 that diverticula of the coelen- 

 teron and of the mouth 

 extend in the direction of 

 each of the tentacles. A 

 stellate arrangement results, 

 the mouth being in the 

 form of a cross. This cor- 

 responds exactly with the 

 condition in the medusa, 

 especially in young speci- 

 meus (PI. XXXIII, fig. 

 19), in which the twisting 

 which in older individuals 

 obscures somewhat the true 



relation of parts has not yet ing agreement in plan between oral lobes 

 , , , T .1 il ' and radial canals, 



taken place. In the three- 



or five-tentacled hydra the number and arrangement of the oral 

 lobes corresponds with the number and arrangement of the tenta- 

 cles. Fig. 13, PI. XXXII, represents a polyp with five radial jiarts, 

 in which one lobe of the mouth is bifurcated. This condition is 

 very similar to that frequently met with in adult medusre (text fig. 

 1). The whole aspect of the hypostome of the Gonionema polyp 

 is very similar to that of the manubrium of the young medusa. 

 The ectoderm at the edges of the mouth becomes thickened and 

 armed with nematocysts, which have by this time made their ap- 

 pearance, in a manner to be described later, over a large part of 



Fig. 1. 

 Abnormal five-parted medusa, show- 



