l>(54 



NATURE STUDY IN SCHOOLS. 



This wonderful process of multiplication is called alternation of genera- 

 tion and occurs in quite a number of animals. 



Another class of hydroid polyps which live in the tropics secrete a stony 

 base. This stony base, once considered a coial, (see fig. 135 C ) if examined 

 with care, will be found to be filled with small hcles systematically arranged. 

 That is, there will be found one large hole with five email ones around it. 

 See fig. 130. From the small holes grow polyps which, being provided with 



Fig. 133. 



A. Eight-armed, fringed Folyp of a Gorfonia, Plexaura. B. Dissection of an Aetini a 

 essentially similar to a eoral polop; c. c, c. arms or tentacles; A, mouth, below which is the cavity, 

 cf the stomreh; D, body. B. opened tentacle, the arrow showing direction of water into the inter- 

 spaces, k, k, the water flowing from one to the ether, through the openings, i, i. 



darts, procure food for themselves and the cential polyp, which is the repro- 

 ductive polyp. This, at certain times, droops its tentacles, and from what 

 was the mouth emerges a saucer-like disk. Beneath this grows another 

 disk, followed by others until quite a pile has been produced. Then the top 

 disk, being now provided with projections, similar to those whieh we found 

 in the young of the bell polyps, drops off and swims away to pioduce a jelly 

 fish. This jelly fish produces eggs that, unelergoing the usual transformation, 

 become polyps like the original millepore polyp. All the other disks drop 

 off in turn and when the last is gone the polyp brings its tentacles up and 

 assumes its life as before. See fig. 129, A being the polyp, B thie saucer- 

 like disk. This is also alternation of generation. 



The millepores are very common in the tropical waters and frequently 



