116 CELENTERATA. 
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 ccelenteron 
it is said to be digested partly by an zxter-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 zvtra-cellular process, the particles of food 
being taken into the endoderm-cells by their amceboid 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-wall 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 archiccele 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 ccelenteron. 
The following special points in Hydra should be noted :— 
. The two-layered body. 
. The axial symmetry and sedentary habitat. 
. The nematocysts and simple hermaphrodite sexual organs. 
. The asexual reproduction by budding. 
. The intra-cellular and inter-cellular modes of digestion. 
. The protected development and formation of hypoblast by 
multipolar ingression. 
Hydra belongs to the phylum CCZELENTERATA 
because its body is didermic or formed of two layers, and to 
the class Hyprozoa because its mouth leads directly into a 
stmple coelenteron. 
AnBWN | 
