THE CCELENTERATES 203 



digestive cells are large, with muscle fibrils at their base and 

 whiplike threads, called flagella, or fingerlike processes, called 

 pseudopodia, at the end which projects into the central cavity. 

 The flagella create currents in the central cavity and the pseudo- 

 podia capture solid food particles. The glandular cells are small 

 and without muscle fibrils (Fig. 120, F). 



Between the ectoderm and entoderm is an extremely thin 

 layer of jellylike substance called mesoglea. 



Digestion. Digestion takes place in the central or gastro- 

 vascular cavity (Fig. 119, gv.c) and probably also within the 

 entoderm cells. The gland cells of the entoderm secrete a 

 fluid into the gastrovascular cavity. This fluid dissolves the 

 food. Digestion is aided by the currents set up by the flagella 

 of the entoderm cells and by the churning resulting from the 

 expansion and contraction of the body. Part of the food is 

 evidently engulfed by the pseudopodia of the entoderm cells 

 and undergoes intracellular digestion. The dissolved food is 

 absorbed by the entoderm cells; part of it, especially the oil 

 globules, is passed over to the ectoderm, where it is stored 

 until needed. 



Reproduction. Hydra reproduces asexually by budding and 

 by fission, and sexually by the production of eggs and sperma- 

 tozoa. Budding (Fig. 119, b) is quite common, and may easily 

 be observed in the laboratory. The bud appears first as a 

 slight bulge in the body wall. This pushes out rapidly into a 

 stalk, which soon develops a circlet of blunt tentacles about its 

 distal end. When full grown, the bud becomes detached and 

 leads a separate existence. 



Fission is less common. The distal end of the animal divides 

 first; then the body slowly splits down the center, the halves 

 finally separating when the basal disk is severed. 



The processes concerned in sexual reproduction are the pro- 

 duction of spermatozoa and eggs, the fertilization of the egg, 

 the development and hatching of the egg, and the growth of the 

 young larva. The spermatozoa arise in the testis (Fig. 119, 



