HYDRA 



89 



-EL 



MY. 



A layer of cells arranged side by side is what is technically known as 

 an epithelium. A cell of the type just mentioned is termed a myo- 

 epithelial cell,, for not only does it form with its neighbours an epithelium, 

 but in the presence of its contractile tail it represents an early stage in 

 the evolution of a muscle-cell. In the case of the ectoderm the myo- 

 epithelial cells are so arranged that the contractile tails run lengthwise, 

 so that when they contract the Hydra shrinks up into a short squat form 

 (Fig. 32, B). The tails lie in close contact with the outer surface of the 

 mesogloea and in transverse sections under a 

 very high magnification they look like irregular 

 tags projecting from the mesogloea (Fig. 33, m.t). 

 The myo-epithelial cell of the ectoderm (Fig. 

 34, A) is normally almost filled by a large fluid 

 vacuole, the protoplasm being so distended as to 

 form merely a thin wall surrounding the vacuole 

 and containing the nucleus embedded in its 

 .substance. At the extreme outer end of the 

 cells the protoplasm is slightly condensed so as 

 to form a protective cuticle over the external 

 surface of the Hydra. On the flattened base of 

 the Hydra the myo-epithelial cells are without 

 cuticle and in place of a large fluid vacuole their 

 protoplasm contains numerous droplets of 

 secreted material a sticky cement which is 

 extruded at the outer end of the cell and helps 

 to attach the Hydra to the substratum. Here 

 we for the first time meet with a gland-cell a 

 cell specialized for the formation of some par- 

 ticular substance or secretion which is passed 

 out from the body of the cell to serve some 

 particular function. 



The spaces between the tapering inner ends of the myo-epithelial 

 cells are occupied by comparatively undifferentiated rounded interstitial 

 cells (Fig. 33, i.c). These are cells which have remained as it were 

 in a young condition, not having developed the various peculiarities 

 characteristic of the myo-epithelial cell. Certain of these cells are destined 



give rise to very remarkable cells termed cnidoblasts which play an 

 iportant part both in defence and in the capture of food. 



The fully developed cnidoblasts are most numerous in the tentacles. 

 Each (Fig. 35, A) is a somewhat oval-shaped cell prolonged into a stiff 

 protoplasmic hair called a cnidocil (en) which projects freely beyond 



FIG. 34. 



Isolated myo - epithelial 

 cells of Hydra. A, From 

 the ectoderm ; B, from the 

 endoderm. (From Graham 

 Kerr's Primer of Zoology.) 

 FL, Flagellum; MY, con- 

 tractile strand. 



