58o THE POPULAR SCIENCE MONTHLY. 



the body, throiigh. a moutli wMdi is monnted upon a short, flexible 

 proboscis, and is surrounded by a circlet or crown of long, elastic 

 tentacles, radiating out in all directions around tlie mouth, and 

 fringed by a poisoning apparatus of microscopic darts, which 

 kill all the small animals which venture within the sweep of the 

 tentacles. The food that is thus captured is conveyed to the 

 mouth, and is swallowed and digested. 



As the colony grows and the feeding members become numer- 

 ous enough to store up a stock of nutriment and to bear the bur- 

 den of a few non-productive parasites, hydras like h in the figure 

 are produced. They are the fighting members, and have neither 

 mouths nor stomachs, but each consists of an enormously elon- 

 gated body, which ends in a battery of poison-darts, which is com- 

 parable to a circlet of undeveloped tentacles. The entire body of 

 one of these fighting hydras is practically equivalent to a single 

 enormous tentacle, although comparative anatomy shows clearly 

 that it is not a tentacle, but that it corresponds to the whole 

 body of a feeding hydra, tentacles and all, rather than to a single 

 tentacle ; that it is actually a hydra which has, during the evolu- 

 tion of the species, lost its mouth and stomach, and its power to 

 capture and swallow food, and has become specialized for defense. 

 These long, slender, outstretched bodies project far beyond the 

 other members of the colony, and their poison-batteries wave in 

 all directions over the heads of the feeding hydras. The shock 

 of contact with them is either fatal or violent enough to paralyze 

 any intruder, or to cause it to beat a hasty retreat. 



As the community gains in numbers and strength, buds of a 

 third sort are produced from the root, and become the reproduc- 

 tive hydras or hlastostyles, which are shown at c in the figure. 

 They are much like the feeding hydras in shape and in general 

 structure, but the tentacles remain rudimentary throughout their 

 life ; they have no mouths, and their capacious stomachs do not 

 open to the outer world, although their walls vigorously assimi- 

 late the food which flows into them through the roots. 



As soon as the blastostyle is fully grown, a circlet of buds 

 grows out from its body, just below its rudimentary tentacles. 

 These buds soon acquire an organization which is very different 

 from that of any of the forms which have been described, and, de- 

 veloping organs of locomotion, are ultimately detached from the 

 hlastostyles, and are set free to begin their independent life as 

 solitary, swimming jelly-fish, like those which are shown at d in 

 the figure. 



The active jelly-fish is as different from all the members of 

 the hydroid colony as a butterfly is from a caterpillar. When 

 fully grown it is vastly larger than a hydra, and it has a well- 

 developed swimming apparatus, which is under the control of a 



