HYDRA 



315 



body; they are appropriately called epitheliomuscular cells. Each 

 of these cells consists of a polyhedral outer or epithelial portion and a 

 basal portion which is drawn into one or two long, slender fibrils 

 extending in a direction parallel to the length of the animal. 

 These cells contract to shorten the length of the animal. Interspersed 

 occasionally among these cells are the larger cnidohlasts in which 

 develop the nematocysts, stinging cells or nettle cells. These are dis- 

 tributed over all the body except the basal disc, but they are much 



TENTACLE 

 MOUTH 



TESTI S 



GASTRO- 

 VASCUI_AR 

 CAVI TV 



ECTODERM 



MESOGUOEA 



EINDODERM 



OVARY 



Fig. 111.- 



-Diagrammatic longitudinal 

 and typical cell layers. 



BASAI_ DISC 



section of hydra, showing mature gonads 

 (Drawn by Titus C. Evans.) 



more numerous near the distal part of the column and on the ten- 

 tacles. The nematocysts are usually ^contained in little raised tuber- 

 cles in the ectoderm. Each tubercle contains a large barbed one and 

 several of a smaller variety. Four different kinds have been de- 

 scribed. Since the lar^ge barbed type is the most conspicuous, it will 

 be described here. Within the cnidoblast it is principally a sac of 

 fluid within which is inverted a stalk with some barbs and a coiled 

 thread attached. Projecting out of the superficial surface of the 

 cnidoblast is a triggerlike process called the cnidocil, which when 

 chemically stimulated causes the cnidoblast to discharge the nemato- 



