54 



ZOOLOGY 



mentary tentacles may occur on the gonophore. As stated 

 above, the medusoid persons in Tubularia do not break away 



or develop further. The gener- 

 y ative organs are found on the 



walls of the manubrium (Fig. 39, 

 II.), the medusoid persons bearing 

 either male or female cells, but 

 not both. 



In those species in which 



Pig. 40. — Diagrams of the forma- 

 tion of Medusae by tudding. 



The ectoderm and endoderm are ^^ medusoid person is completed, 

 represented by the same shadmg ^ , -, 



as in Fig. 39. The Coeienteron other Structures develop ; 



is black. After Korschelt and ^^^^^^ ^^^ ^^^^^ ^^^^^^ 



Heider. 

 I. — Commencement of bud- 



II. — Thickening of ectoderm and 

 cupping of endoderm. 



ten- 

 arise 



round the edge of the umbrella, 

 a muscular fold, the velum, is 

 formed inside the rim of the um- 

 brella (Fig. 40, IV.) and functions 

 as a swimming organ, and the 

 medusa breaks away and swims 

 freely through the water. The 

 existence of these two phases in 

 the life-history of one organism 

 has rendered necessary a double 

 classification. The order to which 

 Tubularia belongs is the Gymno- 

 blastea-Anthomedusae : the first 

 word referring to the nature of 

 the hydroid form, the second to 

 that of the medusoid. 



Order 1. 

 Gymnoblastea-Anthomedusae. 



"^TtyTn'^^Sotom™^"'"* '^'" CHARACTERISTICS.— rA«S6 Hydro- 



zoa have probably always a 

 hydroid stage, which is very various in appearance, sometimes 

 non-colonial, but more usually the hydroids are united into 

 colonies. The ectoderm may form a perisarc, but this 

 is never continued into a cup or hydrotheca surrounding 



