6Q HYDROZOA. 



tubes, or stolons, are sent forth, from which fresh 

 gemmae spring up, it may be, to detach themselves, 

 and so one or several large colonies become 

 formed, all the produce of a single fertilised ovum. 

 For years the hydrosoma may continue in this 

 stage, undergoing no further development. But 

 under certain conditions, similar, perhaps, to those 

 which determine the formation of reproductive 

 organs in the Hydra, a new and striking series 

 ot changes is inaugurated. First, each Hydra- 

 tuba elongates, increasing somewhat in size. 

 Then, from just below the tentacles to within a 

 short distance of the proximal extremity, a succes- 

 sion of transverse markings begin to appear, 

 which quickly take on the aspect of circular con- 

 strictions (g). When the organism was first dis- 

 covered in this condition by Sars, he, thinking it 

 a new animal, called it " Scyphistoma." The same 

 naturalist, observing the Scyphistoma at a still 

 later stage, with the constrictions more strongly 

 marked, and the several segments included between 

 them cleft and lobed around their marmns, gave 

 it, from its resemblance to an artichoke, the name 

 of Strobila (h). Still further do the constrictions 

 deepen until the Strobila becomes not unlike a 

 pile of cups or saucers. The marginal tentacles 

 then disappear, but a new row arises in their stead 

 from the summit of the short, undivided, proximal 

 extremity (i). The disc-like segments above the 

 tentacles gradually fall off, and, swimming freely 

 by the contractions of the lobed margin which 

 each presents, have been described by Eschscholtz 

 as true Medusidce, under the generic title of 

 Ephyra (/o). But each Ephyra soon acquires a 

 nutritive system, lithocysts, tentacles, and genera,- 



