212 



CCELENTERATA. 



beset with eight meridional rows of vibratile plates, which, working 

 like oars, serve for locomotion (fig. 155). 



The body parenchyma in the Sponges consists principally of 

 amoeba-like cells, which frequently bear flagella, but which never 

 produce stinging threads. In the Cnidaria (Polyps and Medusae), 



in certain cells the 



peculiar struc- 



tures known as 



thread cells (fig. 



1 56)are developed. 



They consist of 



small capsules 



filled with fluid. 



and containing a 



sharp-pointed, spi- 



rally coiled thread; 



they are developed 



in cells which may 



be called cnido- 



blasts. Under cer- 



tain mechanical 



conditions, e . g . 



under influence of 



the pressure pro- 



duced by contact 



with a foreign 



body, these cap- 



sules burst, and 



the thread is sud- 



denly protruded, 



and either fastens 



on to the cause of 



disturbance 



or 



FIG 

 plumosa (after Chun). 



mouth. 



pierces it, carrying Fio. 156. Nemutocysts and 

 cnidoblasts of Siphonophora. 

 a and b, with the cnidocil 

 of the cell, c to e, Nemato- 

 cysts with evaginated thread. 



t a part of 



the fluid contents 



of the capsule. In 

 many parts of the body, and especially on the tentacles, which serve 

 for the capture of prey, these small microscopic weapons are collected 

 in masses, and are often united in a peculiar arrangement to form 

 batteries of thread cells. 



