SUB-CLASSES LUCERNARIDA, GRAPTOLITID^J, ETC. 77 



Sea-nettles or Sea-blubbers. Every sea-side visitor is familiar with 

 the great circular discs of jelly which are left upon the sands by 

 the retreating tide during the summer months ; and many must 

 have noticed on a calm day the large transparent discs of these same 

 creatures slowly flapping their way through the water. Not a few, 

 too, must have learnt by painful experience that some of these 

 singular organisms have the power of stinging most severely, if 

 incautiously handled. The forms included under the old name of 

 " covered - eyed " Jfedusce differ considerably from one another in 

 their nature, and even in their structure, though they all present, in 

 spite of their usually greater size, a decided resemblance to the naked- 



Fig. 42. — Development ot Aurelia^ one oi the Lucernarida, a Ciliated free-swimming 

 embryo, or " planula " ; 6 Hydra-tnba ; c Hydra-tuba in which fission has consider- 

 ably advanced ; d Hydra - tuba in which the fission has proceeded still further, 

 and a large number of the segments have been already detached to lead an in- 

 dependent existence. 



eyed Medusce already described. Some of the covered-eyed Medusw 

 produce eggs which are developed into organisms resembling them- 

 selves ; but most of them are now known to be nothing more than 

 the free-swimming reproductive buds of minute rooted Hydrozoa. 

 It will be sufficient here to describe shortly the life-history of one 

 of the more remarkable forms of this section. 



If we commence with the young form of one of these singular 

 animals, we find that the egg gives origin to a little microscopic 

 ciliated body, which swims about freely by means of the cilia with 

 which its surface iscovered (fig. 42, a). This little body, on finding 

 a suitable locality, fixes itself by one end, and develops a mouth and 

 tentacles at the other, when it is known as a "Hydra-tuba" (fig. 42, 



