Page:Popular Science Monthly Volume 33.djvu/598

580 the body, through a mouth which is mounted upon a short, flexible proboscis, and is surrounded by a circlet or crown of long, elastic tentacles, radiating out in all directions around the 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 numerous enough to store up a stock of nutriment and to bear the burden of a few non-productive parasites, hydras like b in the figure are produced. They are the fighting members, and have neither mouths nor stomachs, but each consists of an enormously elongated body, which ends in a battery of poison-darts, which is comparable 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 evolution 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 reproductive hydras or blastostyles, 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 assimilate 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, developing organs of locomotion, are ultimately detached from the blastostyles, 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