688 HYDROIDS, 



1851,) says she "hardly dared to breathe while looking at them, 

 for fear she should blow away their precious lives ; it was most 

 interesting to watch them as they flung about what seemed their 

 fire-like white arms, like microscopic opera dancers or wind- 

 mills ; and most curious to mark their complexions and contor- 

 tions, all the twistings and twirlings, and flingings and wringing? 

 of these curious little creatures." 



THIRD ORDER. HYDROIDS, (Gr. f %*, hydra.) 



This order contains animals, some of which have, and others 

 have not the coralline matter. In these the internal cavity is tu- 

 bular ; some of them have the power of moving from place 

 to place. The coral or hard material varies in different genera, 

 sometimes merely showing itself in calcareous granules diffused 

 through the body. 



The eggs of the Hydroids hang in bunches externally from the 

 lower end of the upper cavity, in graceful forms and sometimes 

 beautifully colored. Prof. Dana divides this order into four fam- 

 ilies. 1. Hydrida, not coralligenous ; 2. SertutandoB, with cor- 

 neous coralla ; 3. CampanularidcR, with corneous bell-shaped 

 corolla ; 4. Tubularida, with coralla tubular and corneous. 



I. HydridcE. It will be noticed that hitherto our attention has 

 been given to marine radiates. This family comprises minute 

 fresh water polyps which are soft and naked. Numbers of them 

 are seen often in stagnant pools and ditches, clustering upon 

 aquatic plants, etc. Their structure consists of a fleshy tube, 

 the aperture of which serves as a mouth to receive and exclude 

 food ; it is bordered with from six to eighteen extremely flexible, 

 thread-like arms. These are so fine as to appear like gossamer, 

 and may be stretched six or eight inches. The Chart names the 

 Green Polyp, H. viridis, (Plate XVIII. fig. 14,) and the Yellow 

 Polyp, H.fusca. When in search of prey, the Hydra permits 

 its arms to float loosely through the water, and thus succeeds in 

 obtaining a supply of food. If any of the minute crustaceans 

 or aquatic insects but touch one of these tentacles, the thread is 

 suddenly "thrown into cork-screw coils," other threads are also 

 coiled around the victim, and it is soon borne quite motionless, to 

 the mouth. At the bottom of the fleshy sac is a saucer-shaped 

 body, " in the center of which is a small, oval, solid body, bear- 

 ing on its summit a calcareous dart, pointed al the extremity, and 

 bifid, or sagittate at its base." The darts are thrust out with 

 force, ejecting, as is thought, a subtle, poison at the same time ; 



