COELENTEHATA HYDROZOA. 139 



are much larger than the ordinary cups in which the 

 polypites reside. These buds, being destitute of mouth 

 and tentacles, obtain their nourishment from the cceno- 

 sarcal fluid, which permeates the tubes of the polypary. 

 In these cells ova are formed. These, when hatched, 

 become oval-shaped, and swim about for a time by the 

 action of cilia, with which they are covered. At length 

 they attach themselves to some solid object, lose their 

 cilia, and form fresh colonies by budding. In the sertu- 

 larians, the generative buds are not detached, as in the 

 club-hydras. 



The extinct Graptolites, found so abundantly in the 

 Silurian rocks, closely resemble the Sertularia. It is 

 believed, however, that they were free-swimming, instead 

 of being permanently attached like the Sertularia. 



202. The LucernaridsB (Lat. lucerna, a lamp), or 

 lantern-hydras, are represented in our seas by one 

 species, which has an umbrella or cup-shaped body of a 

 gelatinous structure. It attaches itself, by the base, to 

 sea-weeds ; but can remove itself at will, like the fresh- 

 water hydra. It is able to swim about, motion being 

 produced by the alternate contraction and expansion of 

 its umbrella. Tufts of short tentacles are placed, at 

 intervals, round the margin of the cup. There is a single 

 polypite in the centre, the mouth of which is surrounded 

 by four lobes. Ova and sperm cells are formed within 

 the body itself, there being no generative buds. 



203. A little animal belonging to this group, about 

 half an inch long, denominated the hydra-tube, from 

 its resemblance to the fresh-water hydra, forms large 

 colonies by budding. The polypites are usually without 

 reproductive organs. In soine cases, the hydra-tube 

 becomes lengthened, and surrounde'd by a number of 

 grooves. These grooves deepen, and acquire notched 

 margins. In this stage, the polypite has the appearance 

 of a pile of minute cups or saucers. The tentacles which 

 originally surrounded the mouth disappear, and a new 

 set sprout out near the baso of the hydra-tube. These 



