xvi TABLE OF CONTENTS 



LESSON XXI. 



I'AGE 



NITELLA . 206 



LESSON XXII. 



HYDRA ; 221 



LESSON XXIII. 



HYDROID POLYPES 237 



BOUGAINVILLEA, &C 237 



DIPHYES 250 



PORPITA 253 



^LESSON XXIV. 



SPERMATOGENESIS AND OOGENESIS 255 



THE MATURATION AND IMPREGNATION OF THE OVUM .... 259 

 THE CONNECTION BETWEEN UNICELLULAR AND DIPLOBLASTIC 



ANIMALS 264 



LESSON XXV. 



POLYGORDIUS 271 



LESSON XXVI. 

 POLYGORDIUS (continued} 293 



LESSON XXVII. 



THE GENERAL CHARACTERS OF THE HIGHER ANIMALS .... 307 



THE STARFISH 309 



THE CRAYFISH 314 



THE FRESH-WATER MUSSEL 32O 



THE DOGFISH 324 



