20 MOKI'IIOLCHiY OF FXYERTEBRATE TYPES 



small so-called interstitial cells, genital cells and nerve cells. 

 The endoderm consists of myo-epithelial flagellated cells and 

 a few gland cells. The former are much larger than the 

 corresponding ectodermal cells and each cell has a long flagel- 

 lunt on the surface facing the cavity of the polyp. Their 

 contractile processes form the system of circular muscular 

 fibres. The hypostome is different from the rest of the hy- 

 drunth head in this respect that both circular and longitu- 

 dinal muscle fibres are more numerous, that genital cells are 

 absent and that among the cells of the ectoderm sensory cells 

 are found. The structure of the tentacles is different in several 

 respects. The tentacles are solid and the core of the tentacle is 

 formed by a single row of large endodermal cells. They have the 

 shape of short cylinders and their protoplasm is quite vacuolated. 

 Circular muscle-fibres are absent. The epithelial layer of the 

 ectoderm consists of myo-epithelial cells, sensory cells and nema- 

 tocysts. The subepithelial layer contains interstitial cells, 

 nerve cells and cnidoblasts or mothercells of the nematocysts. 

 The nematocysts or nettlecells are arranged in batteries at the 

 end of the oral tentacles and in the swellings of the basal ten- 

 tacles. A nematocyst has the shape of an ellipsoid. It has an 

 outer capsule with a short spine or cnidocil at the free end close 

 to the apical opening of the capsule. This opening is closed by a 

 plug. Inside the outer capsule is an inner capsule containing a 

 spirally coiled up filament. The filament is a hollow tube and its 

 wall is continuous with the wall of the inner capsule. \Yhen the 

 cnidocil comes into contact with an object or when the polyp 

 receives a chemical stimulus the nematocyst "explodes," ;'. e., 

 tin- filament is suddenly everted through the apical opening. 

 The nematocysts serve both as organs of defence and for the 

 purpose of killing or stunning small animals which form the 

 food of the polyp. 



Medusome or free swimming medusa. This is the sexual stage 

 in the life cycle of Pennaria tiarella and the sexes are separate. 

 Both sexes look however alike but for a somewhat slenderer 



