HYDROZOA 



25 



the corresponding division of labor. These Hydractin'ia live upon the 

 surface of the shells of sea-snails or whelks, which are inhabited by hermit 

 crabs, and afford another good example 

 of symbiosis. The Hydractin'ia gets 

 free transportation, aiding it in secur- 

 ing food; it also probably feeds upon 

 minute fragments of the crab's food; 

 while the crab, in turn, is protected 

 from intruders by the stinging cells of 

 the H ydractinia. If the hydroids are 

 in any way torn from the shell, the crab 

 finds another colony, and, tearing it 

 loose from its supporting object, places 

 it upon its borrowed shell. 



Millep'ora alcicor'nis is a species of 

 so-called hydroid "corals" the beau- 

 tiful elk-horn .or stag-horn coral of 

 Florida. The permanent colony num- 

 bers thousands of individuals, which 

 differ in their structure according to 

 their division of labor. Their cal- 

 careous skeletons are a cuticular prod- 

 uct of the ectoderm. 



Another order of the class of 

 Hydrozoa is characterized by a 

 closed float containing air or gas 

 which serves to keep the colony 

 vertical in the water. In the 

 " Portuguese man-of-war " (Fig. 

 16), found as far north as New 

 England, there are suspended 

 from the large float (3 to 12 cm.) 1 

 peacock blue, or, in some cases, 

 orange in color, several kinds of 

 individuals; some of them, many 

 feet in length and armed with 

 nettle cells, capture the food and 

 bear it to the mouth-bearing or 

 nutritive polyps, which digest the 

 food and distribute it to the col- 

 ony. Others, the feelers, are groups of deep blue medusoids re- 

 sembling bunches of grapes, while others, with swimming move- 

 ments, aided by the wind, drive the colony from place to place. 



Fig. 16. A Portuguese man- 

 of-war (Physalia), with man-of- 

 war fishes ( Nomeus gronovii) liv- 

 ing in the shelter of the stinging 

 feelers. Specimens from off 

 Tampa, Fla. (From Jordan and 

 Kellogg, " Animal Life," D. 

 Appleton and Co., Publishers.) 



1 Parker and Haswell. 



