212 



C'CELENTERATA. 



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 (Polypa and Medusas), 



in certain cells the 

 peculiar struc- 

 tures known as 

 thread cells (fig. 

 156 )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 

 pierces it, carrying PIG. 156. Ncuwtocysts and 



Fir,, in". r>i<r>ppe (Hormiphora) \ r .^ rt 'f 4- f cnidoblasts of Biphonophora. 



plumosa (after Chun). O, a and 6, with the cnidocil 



mouth. the fluid Contents of th e cell, c to e, Nemato- 



c .-, cysts with evaginated thread. 



or 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. 



