64 THE AMERICAN MONTHLY [March, 



instead of collecting from such sources you can examine carefull}' 

 and underwater shells and stones, etc., raked or dredged froin 

 deeper levels than vou can reach and you will find other objects. 

 Hvdra is found in the same way in fresh water, and is the only 

 hydroid likely to be found there. It is possible to mistake Bryozoa 

 or Polyzoa for Hydroids, but you can easily distinguish by refer- 

 ence to a text-book of zoology. 



Second. Specimens should be kept alive several days in the 

 laboratory and observed from time to time. Each kind should be 

 kept separate and a few in each glass, not having enough to foul 

 the water perceptibly. The water should be changed at least 

 once a dav without uncovering the specimens, which are too deli- 

 cate to endure exposure to the air without injury. Small portions 

 of the colony, or single zooids, should be placed in a watch-glass 

 in sea-water and examined with low power. It will be possible 

 thus to catch the gonozooids in the act of escaping from the parent 

 Hydroid stock. If single zooids be mounted on a slide in sea- 

 water and covered with a cover-glass, many points in their struc- 

 ture can be seen with a quarter-inch objective, including the cir- 

 culation of food in radial and circular vessels of medusae, and the 

 nettle cells ; the working of the latter can also be demonstrated in 

 the act. For direction for preserving and sectionizing hydrozoa 

 the reader should consult some of the many guides for such work. 



I. Hydra fusca* (figs, i to i6). 



Unlike most coelenterates. Hydra \% not a marine animal, but 

 lives in warm and sluggish fresh waters. It is generally attached 

 to aquatic plants. It is a translucent brownish body, \ inch long, 

 a flexible hollow tube fastened at one end, the base, and terminated 

 at the other with a conical 7nanjibriu7n^ in the centre of which is 

 the mouth. The margin of the manubrium bears several tentacles, 

 long filaments which are in constant wavy motion. In the autumn 

 the tubular body may bear small warty prominences, which are 

 the gonads or reproductive organs ; ovaries nearer the manubrium, 

 and spermaries nearer the base. 



The living Hvdra not only moves its tentacles, waving them to 

 and fro, or extending and shortening them, but can do the same 

 thing with the tubular body, and it is thus able to become very 

 large and long and narrow, or to shorten up into a round speck 

 not much larger than a pin's head. It will do this if it is rudely 

 touched, as with a needle. It sometimes leaves its point of attach- 

 ment and creeps about in the water on its oval end or on its side, 

 and it very persistently seeks the lightest side of any vessel in 

 which it is confined. f If any minute animal (e. g., Ostracod or 



* BiBLioGKAPHY OF Hydra. — ** Parker, EI. Biol., p. 209. ** Howes, Biol. Atlas. Hu.\ley 

 and Martin, Practical Biol., p. 343. ** Lankester, Encyc. Britt., vol. xii, p. 561. Packard, 

 Text Book Zool., p. 52. Haddon, Pract. Embryol,, pp. 1-15. Carpenter, The Microscope, p. 

 786. A. M M. Journal, iv, p. 223 ; iv,p. 64; vii, p. 221. Kleinenberg, Hydra. ** Riverside, 

 Natural History, vol. i, article Hydroids. 



t See Heliotropism, E. B. Wilson, Arrierican Naturalist, vol. 25, i8gi, p. 413. 



