144 CORALS 



features of the group to which a passing reference must be 

 made. 



In many of the colonies it is found that the polyps 

 are not all alike but present two or more different kinds 

 adapted for different purposes. One of the commonest 

 forms of this dimorphism is seen in the two Orders of 

 Hydrozoan corals which will be described in this chapter. 

 It consists in the reduction of the tentacles of the one kind, 

 called the gasterozooids, so that they become httle more than 

 a mouth and digestive tube, and in the loss of the mouth and 

 digestive functions in the other kind, called the dactylo- 

 zooids, which become elongated, flexible, and active for 

 catching food by means of the numerous batteries of 

 nematocysts with which they are armed. 



The second feature of importance concerns the origin 

 and position of the ovaries and testes. These organs are 

 always situated in the outer layer of the body wall, and 

 their products when ripe are always discharged directly 

 into the sea and never pass through the body cavity as they 

 do in the Orders of Coelenterata that have been described 

 in previous chapters. Sometimes these genital organs are 

 found on the body wall of ordinary Hydrozoan polyps, but 

 in many other cases they are only borne by specially modified 

 zooids called the medusae, which become detached from the 

 parent colony and swim away to distribute their sexual 

 products in the open sea. 



The medusae are little jelly-fish having a very different 

 appearance from the sedentary polyps of the colony. They 

 have the form of minute umbrellas with usually a ring of 

 tentacles round the margin, and for a handle a short process 

 called the manubrium, at the end of which is situated the 

 mouth. 



In some cases the medusa undergoes degeneration, losing 

 its principal characters, and never succeeds in becoming 

 detached from the parent colony. The story of this 

 degeneration is one of extreme interest to the zoologist, but 

 it has no bearing on the problems dealt with in this book. 



There are two Orders of the Hydrozoa that may fairly 

 be called Corals. These are the Milleporina and the Styla- 



