240 



C(ELENTERATA. 



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

 <dusae ; no polyp generation. 



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

 polyps without medusas, 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 Anthomedusse and the Tubulariae are connected by an alternation 

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

 lariae. There are three groups Trachymedusae, Narcomedusae, 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 



FIG. 184. American hydrqzoan medusae. (Mostly after Fewkes.) A, Eutima gracilis; 

 B, Hydrichthys mirabilis; C, Obelia; D, Euchilota ventricularis; E, Lizzia grata; F, 

 Turritopsis nutricola; (?, Dipurena strangulata. 



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 medusae have bells 

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

 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 (fig. 90), only occa- 

 sionally developing gonads (fig. 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 



