16 LIME-SECRETING HYDROIDS. 



into sections, each will soon produce a crown of tentacles, 

 and grow into a perfect animal. The Hydra can be 

 turned inside out, and within an hour recover its natural 



FIG. 14. Lasso-cells of Hydra, i. Arm of Hydra, containing cells. 

 2. Cell magnified. 3. Cell after bursting open. 



position ; more marvelous yet, if when so treated it is spit- 

 ted with a pin or needle so that it can not turn, it will eat 

 and reproduce its kind as if nothing had occurred. Some 

 are solitary, while others live in colonies. They repro- 

 duce by budding (Fig. 13, b, b) and by eggs. 



Lime-secreting Hydroids (Mt'tfe/x?ra).Some of 

 the Hydroids secrete lime. They resemble true corals in 

 appearance, and were long considered as such. Under 

 the microscope, it will be seen that the lime secreted is 

 tunneled by numerous canals which in life are rilled by the 

 animal. The polyps are of two kinds, and, in a millepore 

 found at Tahiti, they are in groups, the largest being stout 

 polyps, with four tentacles, a stomach, and mouth ; but 

 the polyps about it, rising from the smaller pores, have no 

 mouth or stomach, but many tentacles, whose duty is to 

 capture food for the short, thick-set polyp between them. 

 Some of the Hydroids, instead of bearing young like them- 

 selves, produce perfect jelly-fishes (Medusa) ; such is the 

 Campanularia (Fig. 15), that throws off, by budding, a free 

 jelly-fish (3), that in turn produces eggs that become, not 

 jellv-fishes, but fixed Hydroids (i). This is called alternate 

 generations. The free-swimming young are often brilliant- 

 ly luminous, presenting a wondrous appearance on dark 

 nights. 



