CCELEXTERA TES. 



19 



it covered with a sort of pinkish moss, which closer study shows to be 

 hydroids, much like the hydra of fresh water ; and yet there are many 

 important differences. The 

 buds excepted, each fresh- 

 water hydra is separate from 

 its fellows, but these forms 

 on the snail-shells (Hydrac- 

 1 in ia they are called) are all 

 united by a common base 

 which covers the shell. In 

 the fresh-water hydra only 

 one sort of individual is 

 found. It eats, and develops 

 the male and female repro- 

 ductive elements. In the 

 Hydractinia colony three dif- 

 ferent kinds are found. One 

 has a mouth, and is much 

 like the hydra ; this form 

 does the feeding for the 

 whole colony. The other 

 kinds have no mouths, and 

 cannot eat. They are merely 

 for reproductive purposes, 

 and between themselves they 

 are distinguished in that one 

 furnishes only eggs, the other 

 only spermatozoa. Having 

 no mouths, how are they 

 nourished ? The answer is 

 simple. The feeding forms 

 have their digestive tracts 

 in connection with those of 

 the reproductive ones, by means of canals in the common base. This 

 feature is common to almost all marine hydroids, and is illustrated 

 diagrammatically in Figure 20. 



On the rock-weed between tide-marks another of these forms, Clava 

 by name, occurs, forming pinkish or flesh-colored patches of considerable 

 beauty when expanded. Even more beautiful are the Tubularian forms, 

 which thrive in tidal pools, where they are never exposed to the air, as 

 are the two forms just mentioned. Our plate represents their general 



Fig. 18. — Portion of a colony of hydroids (Hydractinia) , greatly 

 enlarged. The longer individuals eat for the colony, the others 

 are solely concerned in reproduction. 



