CHAPTER XIII 

 HYDROIDS 



THE study of the neuromuscular organization of the 

 coelenterates has been accomplished chiefly on represen- 

 tatives of the scyphozoa and anthozoa. In both these 

 classes the individual animals are of relatively large size 

 and for this reason scyphozoans and anthozoans have 

 been favorable material for anatomical study as well as 

 for experimental work. In the hydrozoa, on the other 

 hand, the polyps and medusae are generally quite small, 

 even microscopic ; consequently members of this class are 

 much less favorable for investigation. The hydrozoa are, 

 however, generally regarded as more primitive than the 

 other two classes, and hence in a study of the elementary 

 nervous system they deserve attention. This is particu- 

 larly true in consequence of the outcome of some of the 

 more recent work on the higher coelenterates. These ani- 

 mals are provided with a neuromuscular mechanism that 

 is by no means so simple as was supposed by many of the 

 earlier workers. It not only includes independent 

 muscles, but muscles associated with nerve-nets and 

 muscles subject to reflex control. As hydrozoans are sup- 

 posed to be simpler in their organization than the other 

 ccelenterates, it is possible that their neuromuscular or- 

 ganization may partake of this simplicity and thus give 

 some clue as to the evolutionary steps that have led from 

 such conditions as those seen in the sponges to such as 

 characterize the higher coelenterates. 



In carrying out such studies a reasonably large polyp 



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