HYDRA AND CCELENTERATES IN GENERAL 



141 



from the extreme rim. The middle layer, the mesoglea, is in 

 them remarkably thick, and resembles jelly, hence their name. 



Hydra and Jellyfishes Compared. Although the medusie 

 upon superficial examination appear to be very different from the 

 polyps or zooids, they are constructed on the same general 



tcl 



FIG. 66. Diagrams showing the similarities of a polyp (A) and a medusa (B). 

 circ., circular canal ; cc/., ectoderm ; end., entoderm ; cnt.'cav., gastro- 

 vascular cavity ; hyp., hypostome ; mnb., manubrium ; msgl., mesoglea ; 

 mth., mouth ; nv., nerve rings ; rod., radial canal ; v., velum. (From 

 Parker and Haswell.) 



plan as the latter. Figure 66 illustrates in a diagrammatic 

 fashion the resemblances between the polyp (A), and the medusa 

 (B) by means of longitudinal sections. If the medusa were 

 grasped at the center of the aboral surface and elongated, a hydra- 

 like form would result. Both have similar parts, the most no- 

 ticeable difference being the enormous quantity of mesoglea 

 present in the medusa. 



Metagenesis. In some Hydrozoa there are two kinds of 

 individuals belonging to the same species; one of these, in the 

 form of a polyp, gives rise asexually by budding to the second 

 form, the medusa, which produces eggs and sperms. The fer- 

 tilized egg develops into the polyp. The polyp, or hydroid stage, 



