240 



CWLENTERATA. 



(5) Siphonophora. Polymorphic, free-swimming colonies of Anthome- 

 dusas ; no polyp generation. 



From this it is seen that there are medusae without polyp stages and 

 polyps without medusae, so that a true system must take into account both 

 these features. When this is done and life histories are traced it is seen 

 that the AuthomedusEB and the Tubularise are connected by an alternatioa 

 of generations, and the same holds good for Loptomedusae and Campanu- 

 lariae. There are three groups — TrachymedusEe, Narcomedusse, and 

 Siphonophora — without a hydroid stage, and two in which the polyp 

 plays the chief role, the medusa being rudimentary in the Hydrocorallinae, 

 lacking in the Hydraria. The hydroid polyps are usually but a few 



Fia. 184.— American hydrozoan medusse. (Mostly after Fewkes."! A, Eittima gracilis; 

 B, Hydricidlqis mir<ihilis; C, Obelia; D, Etichilota ventricularis; E, Lizzia grata; F, 

 Turritopais nietricola; G, Dipurena strangutata. 



millimetres or fractions of a millimetre in size, but the huge Monocaulis 

 imperator, of the deep seas, a yard in length, forms an exception. The 

 colonies are usually only a few inches in extent. The medus» have bells 

 varying between a millimetre and a few inches in diameter, reaching in 

 JEquoria forskalea a diameter of sixteen inches. 



Order I. Hydraria. 



Until recently only the cosmopolitan species of Hydra were known. 

 During most of the year they reproduce by budding (tig. 90), only occa- 

 sionally developing gonads (flg. 172). The eggs remain in connexion 

 with the mother during segmentation, and later form an embryonal shell, 

 protecting them from drought or cold. In this ' encysted stage ' they 

 can be distributed by wind or water birds. These animals formed the 

 basis of the celebrated researches of Trembley on regeneration. He 

 showed that small portions when they contained both body layers could 



