81 



Order Hydromedusae. 



What are the chief Differences between the Hydromedusce 

 and Hydra ? 



The hydroid buds of the Hydromedusce are not liberated, 

 and the result is that colonies (simple or complicated) 

 are formed. There are two principal types of individuals, 

 namely, the fixed hydroid or polyp, which resembles 

 Hydra, and the free-swimming medusoid. 



The tentacles of the hydroid are usually solid, and the 

 ectoderm of the body secretes a horny protective covering 

 known as the perisarc. The medusoid is morphologically a 

 modified polyp. In many instances, it is developed asex- 

 ually as a bud from a specialised reproductive polyp ; and 

 it gives rise to the polyp or hydroid type. 



The life-history frequently shows alternation of genera- 

 tions. 



What is a Zoophyte? Give two Examples. 



Hydroid colonies are termed Zoophytes because their 

 formation is by budding and their appearance plant-like. 

 Obelia and Sertularia are well-known examples. (The 

 Polyzoa are also known as Zoophytes.) 



Write an Account of Obelia geniculata, its Structure and 

 Life-history. 



It is found abundantly on the shore at low-water mark 

 on the fronds of seaweed (Laminaria). Each colony is 

 about an inch in length, thread-like and zig-zag, the little 

 polyps arising at intervals aslant the main stem. 



The ordinary nutritive polyps of the colony have numerous 

 solid tentacles, but otherwise resemble Hydra. From their 

 flower-like appearance they are called hydranths. Each 

 is surrounded by a cup-like extension of the horny perisarc 

 which invests the colony stem. This protective cup, 

 inside which the polyp can be withdrawn, is called the 

 hydrotheca. Where these polyps arise from the stem, 

 another kind of short-stalked polyp is often seen, a tubular 



P 



