SYSTEMATIC /.OOLOGY FOR TEACHEES. 263 



ORDER II. HYDROID POLYPS. 



Differ from the fresh water hydra in living in salt water and, in most 

 species, in living in colonies which form branching moss-like clussers attached 

 to a common stem. Although each individual is small, the whole colony form 

 prominent objects when found growing from bridge piles, or from sea weeds, 

 rocks, etc. 



Hydroid polyps also differ from hydras in other ways. The tentacles 

 cannot be withdrawn or enfolded within the body cavity, but simply shoit- 

 ened. 



The process of propagation is also much more complicated than in the 

 hydra. If we examine some species of branching hydroids, for example, a 

 hell polyp, we shall find that there are buds w r hich appear not from the body 

 of some hydroids, but from the stem. This bud will unfold and, in process 

 of time, produce a bell which is formed like the parent stock. ( See fig. 

 129, a, 1. ) A further examination of the clusters will reveal other buds which 



Fig. 131. 



Portugese Man-of-war. 



are longer and larger than the polyp buds. They are also more opaque, but 

 are still translucent enough for us to see that these buds contain little disks. 

 ( See fig. 129a hi ) These buds, or cases, for such they are, rupture and liberate 

 the disks which are now seen to be provided with little projections on the 

 edges. ( See fig. 129, c. ) They move with these projections and in time 

 produce a small jelly fish, similar to that seen at fig. 129, b. 



These jelly fish produce eggs which, undergoing segmentntion and gas- 

 ticulation, settle down and produce a polyp like the original bell which, by 

 budding, grows up into a branching colony. 



