ALTERNATION OF FORMATIONS 247 



cavity. In this stage the embryo is called a plamda : it 

 swims slowly through the water by means of its cilia, the 

 broader end being directed forwards in progression. 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 (i). 



The attached or proximal end widens into a disc of attach- 

 ment, 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 (K). 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 (L). Soon the cuticle covering the distal end 

 is ruptured so as to set free the growing tentacles (M) : 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. The analogy 

 with Nitella (p. 217), will be at once obvious : in each case 

 there is an alternation of generations, the asexual genera- 

 tions or agamobium (hydroid colony, pro-embryo of Nitella) 

 giving rise by budding to the sexual generation or gamobium 

 (medusa, Nitella-plant), which in its turn produces the 

 agamobium by a sexual process, i.e. by the conjugation of 

 ovum and sperm. 



Two other Hydroids must be briefly referred to in con- 

 cluding the present lesson. 



