THE CCELENTERATES AND SPONGES 131 



layer is known as the entoderm, the outer as the ectoderm. 

 The entoderm lines the large digestive cavity which ex- 

 tends the length of the body, and is continued also into 

 the tubular cavities of the tentacles. Some of the entoderm 

 cells are furnished with lash-like organs, or flagella, whose 

 movements serve to circulate the contained material. 

 The digestive juices poured out by the entoderm cells act 

 on the food in the central cavity, but small particles of 

 food may be engulfed within the cells themselves and 

 digested there. Digestion is, therefore, extracellular, 

 as it is in higher animals, and at the same time intra- 

 cellular, as it is in animals still lower in the scale of life. 

 The undigested residue of the 

 food is ejected through the 

 mouth. 



The ectoderm cells in addi- 

 tion to forming an outer cov- 

 ering for the body are modi- FIG. 108. Cross section of the 

 fied into gland cells, muscle * &S%&S& 



Cells, nerve Cells, Sex Cells and M > mesoglcea; Nc, nettling cell; 

 ii mi i V, vacuole. 



nettling cells. The muscle 



cells endow the Hydra with its extraordinary contrac- 

 tility. At times the animal may be greatly elongated 

 with its tentacles extended into fine threads several 

 inches in length. Touch the animal one or more times; 

 the tentacles will be reduced to mere stubs and the body 

 contracted almost into a ball. There is no central nerv- 

 ous system such as occurs in higher animals, but a scat- 

 tered network of nerve cells whose fine branches or nerves 

 connect with various other cells of the body. 



The most interesting cells of Hydra are the nettling 

 cells which contain oval bodies called nematocysts. The 

 latter consist of a hollow capsule containing a long thread 

 spirally wound up on the inside. In response to certain 



