CCELENTERATA. 



229 



cavity this is seen in development to arise as diverticula from 

 the primitive stomach (enterou). Since such diverticula occur in 

 coalenteratos without becoming independent, one can say that the 

 gastrovascular system consists not only of intestinal portions but, 

 in potentia, of the crelom as well. 



To even a superficial observation the Crelenterata are more 

 clearly animals than are the sponges. The single animals, though 

 often united in colonies, and fixed to some support, are capable of 

 quick and energetic motion. These movements are most striking 

 in the tentacles — long tactile threads, in the neighborhood of the 

 mouth, which have the functions of feeling for food, grasping it, 

 and conveying it to the mouth. The means of killing the prey are 

 the cnidse, nematocysts, or nettle cells (fig. 171), which with rare 



Fig. 171. — Nettle cells of Coelenterata, (After Hertwi^. Lendenfeld, and Hamaiin.) 



exceptions in Protozoa, Turbellaria, and molluscs occur in no 

 other group. These structures, of great systematic importance, 

 are oval vesicles with fluid contents and firm membrane. Each is 

 drawn out at one end into a long tube, so delicate as to appear as 

 a thread (hence an additional name, thread cells). This thread is 

 sometimes armed throughout its length with retrorse hooks, or it 

 may have only a few stronger hooks on its basal portion, whicli is 

 thicker than the rest. In the resting stage the thread is spirally 

 coiled inside the cell. On stimulation the thread is quickly 

 extended ('explosion of cell ') and i)roduces a wound into which 

 passes the irritating fluid contents. Some cffilenterates {e.g. 

 PJiysalia) can produce in this way very painful nettling even in man. 



The nettle cajisulc arises as a plasma product inside a cell. 

 When fully developed the nettle cell extends to the surface and 

 ends with a tactile jirocess (cnidocill) which upon contact stimu- 

 lates the protoplasm and causes the explosion. The cell itself is 

 frequently enclosed by a muscular sheath or a network of muscle 

 fibres. 



Among the ccelenterates both sexual and asexual reproduction 



