264 



COELENTERATA HYDROZOA 



CHAP. 



dz 



form a cup-like receptacle in which the oral extremity of the 

 zooid can be retracted for protection. 



The zooids of the Gymnoblastea present considerable diversity 

 of form and structure. The tentacles may be reduced to one (in 

 Monobrachium) or two (in Lar sabellarum), but usually the 

 number is variable in each individual colony. In many cases, 

 such as Cordylophora, Clava, and many others, the tentacles are 

 irregularly scattered on the sides of the zooids. In others 



there may be a single 

 circlet of about ten or 

 twelve tentacles round 

 the base of the hypo- 

 stome. In some genera 

 the tentacles are 

 arranged in two series 

 (Tubularia, Cory- 

 morpha, Monocaulus), a 

 distal series round the 

 margin of the mouth 

 which may be arranged 

 in a single circlet or 

 scattered irregularly on 

 the hyposome, and a 

 proximal series arranged 

 in a single circlet some 

 little distance from the 

 mouth. In Branchio- 

 cerianthus imperator the 

 number of tentacles is 

 circlets consisting of about two 



Fig. 130. Diagrammatic sketch to show the method 

 of branching of Bougainvillia. gon, Gonophores ; 

 Ilr, hydrorhiza ; t.z, terminal zooid. 



very great, each of the two 

 hundred tentacles. 



The zooids of the hydrosome are usually monomorphic, but 

 there are cases in which different forms of zooid occur in the 

 same colony. In Hydr actinia, for example, no less than four 

 different kinds of zooids have been described. These are called 

 gastrozooids, dactylozooids, tentaculozooids, and blastostyles 

 respectively. The " gastrozooids " are provided with a conical 

 hypostome bearing the mouth and two closely-set circlets of 

 some ten to thirty tentacles. The " dactylozooids " are longer 

 than the gastrozooids and have the habit of actively coiling and 



