HYDROIDS. 251 



They are abundant in the spring. In the middle of 

 summer they lay their eggs and perish. But the eggs 

 do not hatch Medusa; like the parent, but each hatches 

 a httle hydroid which is first free, then afterwards be- 

 comes attached to a shell, seaweed, or stone, and from 

 this little hydroid others branch till a little community 

 of hydroids had grown up, as in Figure 478. From 

 these hydroids bud again the Coryne, Figure 479. 



In some kinds, as Tubularia, Figure 481, the hydroid 

 has a wreath of coronal tentacles, as they are called, 

 a projecting part called a proboscis, and the medusae 

 grow in clusters from just above the coronal tentacles. 



In those called Sertularians and Campanularians, 

 Figure 483, the hydroid has a stem which is covered by 

 a horny sheath, forming a cup around the head. In 

 a fertile cup there are a dozen or moYe hydromedusa, 

 which at length drop out and become free medusa; 

 similar to Tiaropsis, Figure 482. 



In those called Siphonophora, the hydroids exist 

 as free moving communities, each community being 

 made up of individuals of different kinds, yet all so 

 combined as to give the appearance of one animal. 

 The Portuguese Man-of-war, of the Gulf of Mexico, is 

 one of the most remarkable and best known of this 

 sort. It consists of a pear-shaped and elegantly crested 

 air sac, floating lightly upon the water, and giving off 

 from its under surface numerous long and varied ap- 

 pendages. These are the different members of the 

 community, and fill different of^ces ; some of them eat 

 for the whole, others produce medusa buds, and others 

 are the locomotive or swimming members, and have 

 tentacles that stretch out behind the floating com- 

 munity to the length of twenty or thirty feet. 



