126 



THE INVERTEBRATA 



which is columnar in shape and only differs from similar epithelial 

 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 of the 

 body cavity is kept in motion : or these may be retracted and the cell 

 instead puts out pseudopodia to engulf particles of food. In the in- 

 terior of the cell, beyond the border, the food is contained in vacuoles 

 where it is digested, and finally the external border of the cell is pro- 

 duced into permanent cell organs, the muscle fibres or tails already 

 mentioned, in which the cytoplasm can contract with much greater 



nU. 



c.nem 



7nsg 



Fig. 1 06. Diagrammatic transverse section of the body wall of Hydra. 

 Altered from Kukenthal. c.gl. gland cell; c.s. sense cell (the other endo- 

 dermal cells of the musculo-epithelial type may be recognized by possessing 

 either a pair of flagella or pseudopodia); cnc. cnidocil; c.nem. cnidoblasts 

 containing nematocysts in various stages; c.i. interstitial cells; c.n. nerve cell; 

 ect. ectoderm; end. endoderm; msg. mesogloea; m.t. muscle tails of the 

 musculo-epithelial cells. 



force and rapidity than in any other part of the cell. Among the en- 

 dodermal cells, however, some are met with 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 Coelenterata 

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

 cell or cnidohlast. 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. 107) is formed from an 



