THE CCELENTERATES 



55 



closely packed with innumerable lasso-cells, which may be 

 thrown out through openings in the body wall when the 

 animal is attacked. Lasso-cells are also very numerous on 

 the tentacles, which are thus to some extent defensive, but 

 are chiefly active in capturing the crabs and small fish 

 which serve as food. 



The partitions also carry eggs which may undergo the 

 first stages of their growth within the body, and when 

 finally able to swim 

 are sent out through 

 the mouth opening 

 by hundreds to seek 

 out favorable situa- 

 tions, there to set- 

 tle down and re- 

 main. In some spe- 

 cies the young may 

 sometimes arise as 

 buds, as in Hydra 

 (Fig. 27), and in 

 others the animals 

 have been described 

 as splitting longi- 

 tudinally into two 

 equal-sized young. 



51. Corals. The 

 coral polyps also 



belong to this group, showing a very close resemblance to 

 the sea-anemones. In most cases they develop a firm skel- 

 eton of lime, commonly known as " coral/* which serves to 

 protect and support the body. In a few species the polyps 

 throughout life are solitary, and with skeleton comparative- 

 ly simple (Fig. 27) ; but the larger number of species be- 

 come more complex by developing ]-)uds, which retain their 

 connection with the parent, and in turn produce other out- 

 growths with the ultimate result that highly branched 



FIG. 28. Longitudinal section through the body of a 

 sea-anemone, oe, esophagus ; m. /., mesenterial 

 filaments ; r., reproductive organs. 



