70 



Animal Life by the Sea-shore. 



which investment expands at the tip of each branch into a httle 

 cup, wliere a polyp is lodged. Examination with a strong lens 

 will show that these tiny polyps, although of a somewhat 

 simpler structure, are similar in most respects to those which we 

 have described above ; they feed actively on minute organisms, 

 seized with their tentacles, and the digested products are dis- 

 tributed through the colony, the cavities of all the individuals 

 being in communication through the hollow stem. Further 

 examination of the Obelia colony reveals the fact that, in addi- 

 tion to bearing polyps, the 

 branches also carry groups of 

 peculiar little circular buds, 

 which develop into saucer-shaped 

 individuals ; these in course of 

 time break away and swim off 

 as tiny jelly-fishes, leading an 



FIG. 86. TUBULARIA BELLIS AND ENLARGED POLYP OF 



T. LARYNX (after HINCKS) . 



independent existence. When liberated they are no larger than 

 a pin's head ; they are, however, active feeders, and soon 

 grow up, reaching a diameter of about a third of an inch. In 

 Obelia, therefore, we meet with a colony bearing two kinds of 

 individuals, polyps and jelly-fishes, the former merely nutritive 

 in function, the latter reproductive, alone bearing the eggs 

 which it is their duty to distribute as widely as possible. In 

 some Zoopln'tes the jelly-fish stage has been done away with, 

 as in the Sea-firs, Sertularia, and the handsome Tnhularia 

 indivisa (Figs. 86 and 87) ; in the latter the polyps are of 



