PENNARIA TIARELLA McCrady 



Material. P. tiarella is very common along the Atlantic 

 Coast. Medusae may be preserved in either osmic acid or 

 formalin. Polyp colonies may be preserved by any one of the 

 various methods proposed for the fixation of ccelenterata. The 

 student should receive a colony of polyps, a specimen of medusa, 

 a ready made slide of a polyp, stained and somewhat compressed, 

 and a cross-section through a polyp and one through the stem. 



Descriptive Part 



Pennaria tiarella is a typical representative of the class 

 Hydrozoa and has a complete alternation of generations or 

 metagenesis. The polyp is gymnoblastic , i. e., neither the polyps 

 themselves, nor the medusa buds are protected by a covering 

 of the so-called perisarc. 



Hydrosome or polyp colony. The colony is attached to 

 the surface on which it grows, by root-like processes the 

 hydrorhiza. From the hydrorhyza runs a more or less straight 

 stem or hydrocaulus with many alternating side branches which 

 in turn give rise to simple or slightly branched ramuli. The 

 perisarc or the membrane covering of the colony forms ringlets 

 on the stem beyond every branch and at the base of every ramu- 

 lus and ends at the base of every polyp or hydranth. The origi- 

 nal and therefore oldest hydranth of the colony is the one at 

 the top of the main stem. The next oldest hydranth is at the 

 end of the branch nearest to the hydrorhiza. The youngest 

 hydranth of the main stem is always the one nearest the top 



FIG. 4. Obelia sp. from Parker & Haswell's Textbook of Zoology. 



A, portion of a colony with certain parts shown in longitudinal section; 



B, medusa; C, the same with reversed umbrella; D, the same, oral aspect; 

 Bd. i, 2, buds; bis, blastostyle; coe, ccenosarc; ect, ectoderm; etid, endoderm; 



16 



