236 RADIATES: JELLY-FISHES. 



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 hydra 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 more hydro-medusae, 

 which at length drop out and become free medusae 

 similar to Tiaropsis, Figure 482. 



In those called Siphonophorae, the hydroid acalephs 

 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 offices ; some of them eat 

 for the whole, others produce medusa-buds, and oth- 

 ers 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. 



It has recently been discovered by 

 Professor Agassiz, that there are some 

 kinds of Acalephs which produce cor- 

 al similar to that formed by Polyps, 

 described in the following pages, but 

 unlike the latter in having, in the 



cells, a horizontal floor extending 

 Fig. 484 a . Acale- ,, , 



phian Coral. fr m Wal1 to WalL 



