COELENTERATA 



H7 



cells in the higher Metazoa in the fact that it is produced into one or 

 two contractile fibres, which are imbedded in the mesogloea. Such 

 a tissue unit resembles a protozoon in the fact that different parts of the 

 cytoplasm carry on different functions although they are not separated 

 by any partition from each other nor provided with separate nuclei. 

 An endodermal cell of Hydra has an inner border which can be 

 produced into flagella, by 



means of which the fluid ^,, 7/(. cn.f. cn.L ',-./• 



of the body cavity is kept 

 in motion: or these may 

 be retracted and the cell 

 instead puts out pseudo- 

 podia to engulf particles 

 of food. In the interior 

 of the cell, beyond the 

 border, the food is con- 

 tained in vacuoles where 

 it is digested, and finally 

 the external border of the 

 cell is produced into 

 permanent cell organs, 

 the muscle fibres or tails 

 already mentioned, in 

 which the cytoplasm can 

 contract with much 

 greater force and rapidity 

 than in any other part 

 of the cell. Among the 

 endodermal cells, how- 

 ever, some are met with 



Fig. 1 08. Diagrammatic longitudinal section 

 of the body wall of Hydra. From Manton. 

 c. crystal; cb. cnidoblast; en. cnidocil; d.f. de- 

 veloping flagellum; d.n. developing nemato- 

 cyst; e.f. ectodermal muscle fibre; e.i. inter- 

 stitial cell of ectoderm; e.m. ectodermal mus- 

 culo-epithelial cell; en.f. endodermal muscle 

 fibre ; en.i. interstitial cell of endoderm ; /. flagel- 

 lum ; f.v. food vacuole ; g. nerve cell ; g.c. gland 

 cell; i.f. food inclusion; m. mesogloea; n. ne- 

 matocyst; p. pseudopodium ; s.c. sense cell; 

 su. supporting cell. 



of a more specialized type : gland cells which pour into the cavity a 

 digestive secretion (for the preparatory or extracellular digestion), and 

 sense cells, found also in the ectoderm, which are thread-like, with 

 a short projecting process. Both these kinds have no muscle tails. 



A type of cell which is even more characteristic of the Coelentetata 

 (except the Ctenophora) than the musculo-epithelial cell is the thread 

 cell or cnidoblast. Though this would appear to have reached the 

 highest peak of specialization it must be pointed out that within the 

 limits of a single cell many functions are performed and a machinery 

 developed which would be formed from a number of different kinds 

 of cells in the higher Metazoa. A thread cell (Fig. 109) is formed from 

 an undifferentiated interstitial celF: part of the cytoplasm becomes 



^ This is a type of cell which preserves an embryonic character and may 

 develop into germ cells and musculo-epithelial cells as well as cnidoblasts. 



