PHYLUM CCELENTERATA 119 



and habits. The two chief shapes assumed by the Hydrozoa are 

 the hydroid, or polyp, like Hydra and Obelia (Fig. 73), and the 

 jellyfish, or medusa, like Gonionemus (Fig. 74). There are many 

 variations of each of these, and frequently one species may ex- 

 hibit both conditions at different periods in its life-history. 



a. A Colonial Hydrozoon — Obelia l 



Obelia (Fig. 73) is a colonial ccelenterate which lives in the 

 sea, where it is usually attached to rocks, to wharves, or to Lami- 

 naria, Rhodymenia, and other algae. It may be found in low 

 water and to a depth of forty fathoms along the coast of northern 

 Europe and from Long Island Sound to Labrador. 



Anatomy and Physiology. — An Obelia colony consists of a 

 basal stem, the hydrorhiza, which is attached to the substratum; 

 this gives off at intervals upright branches, known as hydrocauli. 

 At every bend in the zigzag hydrocaulus a side branch arises. 

 The stem of this side branch is ringed and is expanded at the 

 end into a hydra-like structure, the hydranth (Fig. 73, A). A 

 single polyp consists of a hydranth and the part of the stalk be- 

 tween the hydranth and the point of origin of the preceding 

 branch. Full-grown colonies usually bear reproductive members 

 (gonangia) in the angles where the hydranths arise from the hy- 

 drocaulus (Fig. 73, A, 8, q, 10). 



The Obelia colony as just described and as shown in Fig. 73, A, 

 resembles the structure that would be built up by a budding 

 Hydra if the buds were to remain attached to the parent and in 

 turn produce fixed buds. 



All of the soft parts of the Obelia colony are protected by a 

 chitinous covering called the perisarc (Fig. 73, A, 6); this is 

 ringed at various places and is expanded into cup-shaped hydro- 

 thecce (Fig. 73, A, 7) to accommodate the hydranths, and into 

 gonothecce (Fig. 73, A, 10) to inclose the reproductive members. 

 A shelf which extends across the base of the hydrotheca serves 

 Co support the hydranth. The soft parts of the hydrocaulus 



s Campanularia is similar to Obelia in most respects. 



