64 THE AMERICAN MONTHLY [MaiTh, 



instead of collectini; from such sources yon can o\;iniinc carciiilly 

 and under water shells atul stones, etc.. raked or dredj^ed from 

 deeper levels than vou can reach and you will llnd other ohjects. 

 llvdra is t"<umd in the same way in fresh water, ami is the only 

 hvthoid likelv to ho tountl there. It is possihle t<> mistake Mryo/oa 

 or Polvzoa for llvdroids. hut yon can easily distinj^uish l)y refer- 

 ence to a text-hook of /.ot)lo«r\ . 



Srct>nt/. Specimens should he kept alive several da\s in the 

 lahoratorv and ohserveil from time to time. ICach kind should he 

 kept separate and a few in each tjlass. not ha\ in>^ enonj^li to foul 

 the water perceptihlv. The water should he chan<;ed 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 colonv. or single zooids. shouKl he placed in a watch-glass 

 in sea-water and examined with low power. It will he possihle 

 thus to catch the gono/ooiils in the act of escaping from the parent 

 Hvilroiil stock. If single /ooids he mounted on a slide in sea- 

 water and co\ered with a cover-glass, many points in their struc- 

 ture can he seen with a quarter-inch ohjectivc, including the cir- 

 culation of food in railial and circular vessels of medusa-, and the 

 nettle cells: the working of the latter can also he demonstrated in 

 the act. For direction for preserving and sectioniziug hvdrozoa 

 the reader shoulil consult some of the manv guides for such work. 



I. IIVDKA FUSCA* (figS. I tO l6). 



Unlike most codenterates. Hydra is not a marine animal, hut 

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

 to aquatic plants. It is a translucent hrownish hodv. \ inch long, 

 a flexihlc holhjw tuhe fastened atone end. the hase.and terminated 

 at the other with a conical »ianNlfr/u/n. in the centre of which is 

 the mouth. The margin u[' the mannhrinm hears several tentacles, 

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

 the tuhular hody may hear small warty prominences, which are 

 the gonads or reproductive organs ; ovaries nearer the manubrium, 

 and spermaries nearer the base. 



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

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

 thing with the tubular l)od} . 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 (jn its oval end or on its side, 

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

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



*BiBUOGMAPHY OP Hydra. — *• Parker, El. Biol., p. 20^. •♦Howes, Biol. Atlas. Huxlev 

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

 Text Book Zool , p. 52. Haddon, Pract. Emijryol , pp. i-is. Carpenter, I'hc Microscope, p. 

 786. A. M .M. Journal, iv, p. 223; iv, p. 64: vli, p. 221. Kleincnborg, Hydra. ♦•Riverside, 

 Natural History, vol. L article Hydroids. 



t See Helioiropism, E. B Wilson, American Naturalist, vol. 25, 1891, p. 413. 



