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which is seen more iilainly in larger Coetenterates, like the 
Jelly-Fish and Sea Anemone. This is a point of resemhlance 
with the Echino<lerms, but otherwise they differ throughoul 
from the Echinoderms and all other higher animals. In the 
latter the alimentary canal is always enclosed In a special cavity, 
the coelom. There is no such cavity in Coelenterates, Their 
body -wall forms at the same time the wall of the digestive 
cavity or coetenteron. The second great difference of the 
Coelenterates from all higher animals is the presence of the 
thread-cells in the former. 
Coelenterates are divided into three classes^ (i) the 
Hydrosoii, including chiefly Hydra and the Zoophytes ; (2) the 
Scyphozou or Jelly-Fish ; and (3) the Antkocaa. including Sea 
Anemones, Corals. Sea-Fans, Sea Pens and their allies. 
HYDROZOA, 
The simplest member of tlie class is the Freshwater 
Polyp Hydra, To its description given above we only have to 
add that it reproduces asexnally and sexnally. The asexual 
method of reproduction or ' budding ' consists in the formation 
of an hollow outgrowth from the sides of its body which deve- 
lops tentacles and becomes in all details like the parent Hydra. 
This outgrowth or ' bud' may become dftached, or may remain 
attached for a considerable tinie to form with other buds a tree- 
like colony. The sexual method consists in the formation of 
eggs in the ectoderm of the Hydni, 
The other Hydrozoa are veiy much more complicated, 
both in structure and in developjiient. They nearly always 
form colonies with two or more sorts of polyps. One kind of 
the polyps remains stertie, and devotes itself merely to feeding 
the colony, whilst the other kind develops into little swimming 
bells resembling Jelly-Fish (so-called ' Craspedote Medusae ') 
which become detached from the parent stock, swim about 
freely and Say eggs from which again fixed tree-like stocks are 
developed. This process is repeated over and over again and 
is called ' alternation of generations.' 
We mention only two or three of the numerous orders 
of this class, those which develop either a horny or a calcare- 
ous skeleton- Those with a horny skeleton are popularly 
called Zoophytes and are frequently mistaken for sea-weeds. 
They form tree-like colonies with horny stalks, and homy cups 
into which the little polyps can withdraw for protection. They 
are common in shallow water, and their dead skeletons may 
