DEVELOPMENT 3 I 7 



rise to a free-swimming Medusa similar in all essential features 

 to the parent. 



In many species, however, the planula larva sinks to the 

 bottom of the sea, develops tentacles, and becomes attached by 

 its aboral extremity to a rock or weed, forming a sedentary 

 asexual stage of development with a superficial resemblance to 

 a Hydra. This stage is the " Scyphistoma," and notwithstanding 

 its simple external features it is already in all essential 

 anatomical characters a Scyphozoon. 



The Scyphistoma may remain as such for some time, during 

 which it reproduces by budding, and in some localities it may 

 be found in great numbers on seaweeds and stones.^ 



In the course of time, however, the Scyphistoma exhibits a 

 ring-like constriction of the body just below the crown of tentacles, 

 and as this deepens the general features of a Scyphomedusa are 

 developed in the free part above the constriction. In time this 

 free part escapes as a small free-swimming jelly-fish, called an 

 " Ephyra," while the attached part remains to repeat the process. 

 In many species the first constriction is 

 followed by a second immediately below it, 

 then a third, a fourth, and so on, until 

 the Scyphistoma is transformed into a long 

 series of narrow discs, each one acquiring, 

 as it grows, the Ephyra characters. Such 

 a stage has been compared in form to a pile 

 of saucers, and is known as the " Strobila." 



The Ephyra differs from the adult in 

 many respects. The disc is thin and flat, fig. 144. —The perisare 

 the manubrium short, the margin of the ^^^^^^^SSVl 



umbrella deeply grooved, while the stato- ramifying in the skeleton 



rhabs are mounted on bifid lobes which tZHT^.'T:! 



project outwards from the margin. The in a macerate.l specimen. 



,.,.,. ,. ,1 n 1 ■ i • X !■ (After Schulze. ) 



strobihsation of the Scyphistoma is a pro- 

 cess of reproduction by transverse fission, and in some cases this 

 is supplemented by gemmation, the Scyphistoma giving rise to 

 a number of buds which become detached from the parent and 

 subsequently undergo the process of strobilisation. 



The Scyphistoma of Nausithoe presents us with the most 



1 For good illustrations of this see Sir J. Dalyell, " Rare and Remarkable 

 Animals of Scotland," vol. i. 1847, pll. 13, 14, 18, 19, 20. 



