v ALTERNATION OF GENERATIONS 321 



plast or morula (p. 200) : ectoderm and endoderm become 

 differentiated, and the ectoderm cells acquire cilia, by means 

 of which the embryo now swims freely in the water. An 

 enteron appears in the endoderm, and in this stage 

 the embryo, which has an elongated form, is known as a 

 planula. It then loses its cilia and settles down on a rock, 

 shell, sea-weed, or other submarine object, assuming a 

 vertical position with its broader end fixed to the support. 



The attached or proximal end widens into a disc of 

 attachment, a dilatation is formed a short distance from the 

 free or distal end, and a thin cuticle is secreted from the 

 whole surface of the ectoderm. From the dilated portion 

 short buds arise in a circle : these are the rudiments of the 

 tentacles : the narrow portion beyond their origin becomes 

 the hypostome. Soon the cuticle covering the distal end 

 is ruptured so as to set free the growing tentacles : an 

 aperture, the mouth, is formed at the end of the hypostome, 

 and the young hydroid has very much the appearance of a 

 Hydra with a broad disc of attachment, and with a cuticle 

 covering the greater part of the body. Extensive budding 

 next takes place, the result being the formation of the 

 ordinary hydroid colony. 



Thus from the oosperm or impregnated egg-cell of the 

 medusa the hydroid colony arises, while the medusa is 

 produced by budding from the hydroid colony. We have 

 what is called an alternation of generations, the asexual genera- 

 tion or agamobium (hydroid colony) giving rise by budding 

 to the sexual generation or gamobium (medusa), which in its 

 turn produces the agamobium by a sexual process, i.e. by 

 the conjugation of ovum and sperm (compare p. 287). 



Hydra and Obelia both belong to the simplest class 

 the Hydrozoa of the phylum Coelenterata : this phylum 



PRACT. ZOOL. Y 



