ii6 . CCELENTERATA. 



processes probably reacting with the elastic mesogloea. 

 The food is ingested and egested through the mouth which 

 thus functions as a mouth and an anus. In the coelenteron 

 it is said to be digested partly by an inter-cellular process, 

 consisting of the reduction to a soluble condition by the 

 digestive fluid discharged from the vacuoles of the endoderm, 

 and partly by an intra-cellular process, the particles of food 

 being taken into the endoderm cells by their amoeboid ends. 



Hydra reproduces, not only by the sexual method, but 

 by the asexual process of budding. A bud is a simple 

 process of the body-w^all which grows outwards, acquires a 

 mouth and tentacles and finally detaches itself from the 

 parent. 



The amoeboid ovum containing a few scattered yolk 

 granules protrudes from the ectoderm of the parent and 

 is here fertilised. It loses its power of movement, becomes 

 spherical, and encysts. The cyst is secreted by the ovum 

 itself and the egg then falls from the parent and remains 

 dormant for several weeks. It segments by total equal 

 segmentation producing a blastula. Certain of the cells 

 then wander into the archicoele cavity to form the hypoblast, 

 which is thus formed by multipolar ingression. Eventually 

 the diploblastic larva escapes from the cyst and elongates. 

 At one end the mouth is formed by rupture of the layers 

 and the other end becomes attached. The two embryonic 

 layers, epiblast and hypoblast, become the ectoderm and 

 endoderm and the archenteron becomes the coelenteron. 



The following special points in Hydra should be noted : — 



1. The two -layered body. 



2. The axial symmetry and sedentary habitat. 



3. The nematocysts and simple hermaphrodite sexual organs. 



4. The asexual reproduction by budding. 



5. The intra-cellular and inter-cellular modes of digestion. 



6. The protected development and formation of hypoblast by 



multipolar ingression. 



Hydra belongs to the phylum CCELENTERATA 

 because its body is didermic or formed of two layers, and to 

 the class Hydrozoa because its mouth leads directly into a 

 simple coelenteron. 



