CCELENTERATA. 151 



the case, the reproductive polyps assume the special- 

 ized type characteristic, of the group, i.e., that of a 

 Medusa, or jelly-fish, while the ordinary polyps are 

 more like Hydra, the simplest form of auiraal in the 

 group. They are therefore spoken of respectively as 

 medusoid polyps and hydroid polyps. 



The animals that exhibit " alternation of genera- 

 tions " afford some of the most complicated problems 

 of morphology ; and there are cases in which it is 

 rather difficult to distinguish between an alternation of 

 this sort and the alternation of forms which is appa- 

 rent in an animal which has a larval stage that looks 

 very different from the adult stage.i 



' It is interesting to compare the state of things we find 

 in many of the Coelenterata with the alternation of genera- 

 tions that happens in plants, the details of which may be 

 sntrnnarized here, since they may not be known to the reader. 

 Those green plants which have vascular stems and leaf-ribs, 

 but do not bear those coloured expansions of modified leaves 

 surrounding the essential organs, which are popularly known 

 as " flowers," are called Vascular Oryptoga/ms ; * they include 

 the mosses and ferns and other allied plants. The ferns, — for 

 instance the Common Bracken Fern, — show the typical form 

 of alternation of generations in plants. The " fern," as known 

 to us, is the asexual generation of the plant, which is produced 

 from a properly fertilized egg, or oosphere (egg-sphere), astht; 

 vegetable egg is called. On the back of the leaf (or frond, as a 

 spore-bearing leaf is called) are developed, in masses called 

 sori, protected by a covering called the indusium-, the spores; 



* Vascular, i.e. oontainiag vessels. The "vessels" of plants are pipes 

 of thickened or woody tissue of various forms, which serve the double 

 purpose of conveying fluid and of supporting the soft cellular tissue. 

 Tissue of this sort does not exist in the lower cryptogams — cryptogams, 

 i.e., plants with a hidden (inconspicuous) flower. 



