20 



COMPOUND HYDRO IDS, 



Development of Discophorce. They multiply by eggs 

 that are deposited in the autumn, and are at first spheri- 

 cal, quickly changing to a pear-shape, and attaching them- 

 selves to rocks or weed. Now, 

 tentacles appear (Fig. 17, i), 

 varying in number from eight 

 to twenty-four, and after eight- 

 een months the pear divides 

 off into disks until we have a 

 pile of scalloped saucers one 

 upon another (2). The high- 

 est one dies, while all the rest 

 break off and swim away, lit- 

 tle jelly-fishes, that in time 

 grow into the gigantic Cyanea 

 or others, as the case may be. 



VALUE. They form the food of 

 some whales and fishes. 



NOTE. The gigantic Cyanea af- 

 fords a home under its curtains for 

 numbers of fishes and several crus- 

 taceans, while in its mouth-folds 

 lives a long, tapering sea-anemone 

 (Bicidiiim parasiticiini), that in many 

 cases mimics in color that of its 

 protector. 



Order III. Siphonopho- 



ra. These are the most beau- 

 tiful of the Hydroids, ah.d 

 are rarely seen in the North. 

 They are free-swimming colo- 

 nies. The Portuguese man- 



o'-war, or Physalia (Fig. 19), is* a mere bubble, seemingly 

 of the finest satin, that floats upon the surface. From the 

 upper portion rises at will a fluted membrane, colored with 

 delicate tints of pink, that is used as a sail ; from the low- 



FIG. 19. Portuguese man-o'-war, 

 Physalia Arethusa. 



