HYDROIDEA. 461 



It is thus seen that we have here another illustration of 

 u parthenogenesis," or u alternations of generations " (see 

 page 329). The egg hatches into a Planulu, which soon 

 becomes a Scyphistoma / the scyphistonia becomes a Stro- 

 bila ; and the strobila breaks up into Epkyrw, which soon 

 become perfectly formed into Jelly-fishes which lay the 



eggs. 



SUB-SECTION IV. 

 THE ORDER OF HYDROIUEA OR HYDEOIDS. 



THE Hydroids are Jelly-fishes which are even more won- 

 derful in their mode of development than those already 

 described. Occurring as they do in many cases, in their 

 early stages of existence, as mere discolored patches on 

 sea-weeds, stones, or shells, or in appearance like little 

 tufts of moss, or miniature shrubs, the untrained eye 

 might well mistake the fact that they are animals. 



But naturalists have shown that these plant-like forms 

 produce medusae buds, which expand into genuine medusie 

 or jelly-fishes. Fig. 694 shows a little cluster of Hydroids 

 attached to sea-weed, and Fig. 691B shows a single indi- 

 vidual of the same, very much magnified, with two of 

 the buds much enlarged, and a third quite prominent. 

 At length each bud becomes detached, and floats away as 

 a free jelly-fish, like Fig. 69-iC, and is then known as Co- 

 ryne ; or, as it was formerly called, Sarsia the latter name 

 from Sars, a Norwegian naturalist, who was one of the 

 earliest investigators of these curious kinds of Jelly-fishes. 



Coryne is almost as delicate and as transparent as a 

 dew-drop, yet it performs varied and rapid movements, 

 contracts and expands its tentacles, catches and devours 



