PENNARIA TIARELLA (A Hydroid). 



Phylum III, CGELENTERATA; class i, HYDROZOA; order i, 



LEPTOLIN^:. 



HABITAT. Pennaria is a marine zoophyte (plant-like animal) 

 occurring in shallow water along our coast. Colonies of it, looking 

 like masses of delicate pink seaweed or moss, often cover the rocks 

 over small areas. 



Technical Note. Place a small portion of a colony, consisting of five or 

 six polyps, in a watch glass full of water and examine with the simple 

 microscope. 



THE COLONY. A colony of these organisms consists of a number of 

 horizontal axes or stems, much like the creeping stolons of some plants, 

 and vertical axes which give off lateral branches in alternate arrange- 

 ment. At the ends of these short lateral branches the separate polyps 

 or hydranths are borne. The vertical axes with all their branches 

 constitute the hydrocaulis, the horizontal axes the hydrorhiza. 

 Each part of the whole colony, except the hydranths, is covered by a 

 tough horny sheath, the perisarc. 



Note that the polyps are not all of the same size. How can you 

 account for this ? Some hydranths may be found which are mere buds, 

 having not yet expanded. The specimen may also have short branches 

 from which the hydranths have been accidentally broken off. Look 

 for ringed constrictions on the stem in certain places. What purpose 

 do they serve ? In most places the perisarc is so transparent that the 

 fleshy continuation of the hydranth down into the stem can easily be 

 seen. This fleshy core of the stem is called the coenosarc. It puts the 

 various polyps of a colony into organic connection. 



Exercise i. Make a drawing of the portion of a colony you have just 

 examined, giving particular attention to the method of branching, 

 position of the hydranths, and arrangement of the tentacles. 



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