HISTOLOGY OF HYDRA 261 



of epithelio-muscular cells : they are large, rather flat, and 

 provided with several longitudinally-disposed muscle-fibres. 

 When the tentacle is contracted each cell is transversely 

 elongated and bulged outwards, giving rise to the transverse 

 wrinkles already noted. When the tentacle is extended the 

 central part of each cell forms a projecting lump, whilst its 

 periphery is thinned out to a thin protoplasmic layer. The 

 central lump contains a large nucleus with a distinct nucleolus, 

 and a variable number, often as many as twelve, of nemato- 

 cysts embedded in the cell substance. Usually there is one 

 sometimes two or three large oval nematocyst occupying 

 the centre of the swelling, and around this are several of 

 the two other kinds of nematocysts viz. cylindrical and small 

 oval. The arrangement can best be understood by an examina- 

 tion of fig. 54, B. It will be noticed that each nematocyst is 

 contained in a cnidoblast which is itself embedded in the 

 epithelio-muscular cell. Each cnidoblast is furnished with a 

 cnidocil projecting through the cuticle, the cnidocils of the 

 cylindrical and small oval nematocysts being longer than those 

 of the large oval form. The cell-bodies of the smaller cnido- 

 blasts are produced internally into a fibril which, on reaching 

 the mesogloea, bends sharply and runs backwards amongst the 

 muscle-fibres. The large cnidoblasts have shorter and less 

 conspicuous fibres. There can be little doubt that these fibres 

 are contractile, and belong to the same category as the muscular 

 processes of the epithelio-muscular cells. 



The ectoderm of the basal disc consists almost entirely of 

 columnar epithelio-muscular cells which are filled with small 

 refringent granules and have no external cuticle. A few 

 interstitial cells may be found among their bases, but no 

 nematocysts. A single cell from the basal disc is shown in 

 fig. 55, B. Its outer half is longitudinally fibrillated and along 

 each fibril is arranged a row of bright refringent corpuscles 

 which are the secretum of the cell. Thus the basal cells are 

 glandular as well as epithelial and muscular. Their function 

 is to secrete a sticky substance by means of which the animal 

 can adhere to a foreign body. Occasionally the basal cells 

 send out pseudopodial projections from their free surfaces, and 

 possibly the animal is able to effect slight changes of position 

 by means of these pseudopodia. 



The endoderm lines the whole of the gastro-vascular cavity, 



