446 



THE WONDER OF LIFE 



Fig. 69. — ^Minute trans- 

 parent free-swimming 

 larva of a sea-ououm- 

 ber or Holothurian, 

 showing transverse 

 bands of cUia (c) 

 and peculiar protrud- 

 ing ' arms ' (a). 



tubae,' about an eighth of an inch in height, with a mouth, 

 gullet, and tentacles. In ordinary conditions this sedentary 

 stage grows larger, and displays a 

 series of transverse annular con- 

 strictions, becoming hke a minia- 

 ture pile of saucers — the strobila 

 stage. Each disc or saucer is 

 separated off in turn as a free- 

 swimming young jelly-fish (or 

 Ephyra), which feeds on micro- 

 scopic organisms, grows rapidly, 

 undergoes certain structural 

 changes, and becomes a sexual 

 jelly-fish. Thus we find that a 

 characteristically free and active 

 animal, the jeUy-fish, includes in its life-history a fixed 

 and vegetative polyp-stage — alternation of generations 

 again (see Fig. 72). 



Echinoderms. — The newly- 

 hatched larvse of sea-urchins, sea- 

 cucumbers, starfishes, and brittle 

 stars are diffusely ciliated two- 

 layered thimble-like sacs — in fact, 

 not very remarkable gastrulse. 

 But they soon become quaintly 

 transformed by the outgrowth of 

 processes and the formation of 

 special bands of cilia into extra- 

 ordinarily shaped larvae, adapted 

 for open sea Hfe. In sea-urchins, 

 for instance, the quaint larva, 

 known as a Pluteus, is often 



Fig. 70. — Minute trans- 

 parent free-swimming 

 larva of a sea-cucum- 

 ber or Holothurian, 

 showing peculiar pro- 

 truding ' arms ' (a) 

 and calcareous plates 



(CP). 



