CCELENTERATA 



207 



are given off from the central sac which distribute the nourishment to the 

 peripheral parts of the Ijody, and thus functionally replace the vascular 

 system of higher forms. Since this gastrovascular system is primarily for 

 nourishment, it is not a body cavity and one cannot say that the ccelen- 

 terates are stomaclrless. On the other hand, the term 'ccelenteron, ' that 

 is, a cavity at once gastric and ccelomic (p. 148), is perfectly defensible, 

 since in many liigher animals which possess a true body cavity (cfx-lom) 

 this arises in development as diverticula from the primitive stomach 

 (enteron) . Since such diverticula occur in ccelenterates without becoming 

 independent, one can say that the gastrovascular system consists not only 

 of intestinal portions but, in potent la, of the coelom as well. 



To even a superficial observation the Ccelenterata 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 processes in the neighborhood of the mouth, which feel for food, 

 grasp it, and convey it to the mouth. The means of killing the prey are the 

 cnida (whence the name Cnidaria for the phylum), nematocysts, or nettle 

 cells (fig. 168). These structures, of great systematic importance, are oval 

 or elongate vesicles with fluid con- 

 tents and firm membrane. Each is 

 drawn out at one end into a long 

 thread-like tube (hence an additional 

 name, thread cells). In the resting 

 stage the thread is spirally coiled 

 inside the cell. On stimulation the 

 thread is quickly extended ('explosion 

 of cell') and produces a wound into 

 which passes the irritating fluid con- 

 tents. Some ccelenterates (e.g., 

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

 The nettle capsule arises as a plasma product inside a cell. When 

 fully developed the nettle ceU extends to the surface and ends with a 

 tactile process {cnidocill) which, upon contact, stimulates the protoplasm 

 and causes the explosion, the thread being everted like the finger of a 

 glove. The cell itself is frequently enclosed by a muscular sheath or a 

 network of muscle fibres. 



Among the ccelenterates both sexual and asexual reproduction may 

 occur, the latter usually by budding, more rarely by division. Sexual and 

 asexual reproduction can be combined in the same species, producing an 

 alternation of generations. 



Fig. 168. — Nettle cells of Ccelen- 

 terata (after Hertwig, Lendenfeld, and 

 Hamann) . 



