Systematic Course. 5 
rescent appearance to the water, and Globigerina, which. secretes 
minute calcareous shells found in vast numbers at the bottom of some 
parts of the ocean, where they constitute a fine mud-like substance 
known as Ooze, also Nummulites, whose shells are largely found in 
limestione rocks (see specimens), and Radiolaria with minute flint shells, 
which are often most wonderfully sculptured. An easily procured and 
convenient Protozoon for the students to examine is the Opadina parasitic 
on the frog (see practical work), 
Il.—METAZOA. 
A.—CCSLENTERATA. 
The Ccelenterata comprise all the jelly-fish, sea-anemones, coral 
polyps and sponges, the commonest fresh-water representative bemg 
Hydra, which is sometimes to be found in tanks in India. The Ceelen- 
terata differ frém the Protozoa in having the body made upon a number 
of cells.* The most important point to notice is that, in its essential feae 
tures, the.body is little more.than a bag. Food is taken into the 
interior of the bag, which thus becomes the digestive tract. The cells 
of which the body is composed each absorbs its nutriment direct, so there 
is no need for blood-vessels or for any space between the digestive tract 
and the outer layer of the body. The Coelenterata thus differ from al] 
the groups of higher animals (Coelomata), which have, at one time or 
other of their existence, a well-marked space (body cavity) in which the 
digestive tract is suspended. 
Coral is the calcareous matter which is secreted by colonies of coral 
polyps, each of which is extremely simple im structure. The complicated 
structure presented by the coral itself is chiefly dué to the process 
of budding, by which the original animals surround themselves with 
out-growths, each of which becomes a perfect polyp and secretes coral, 
though it remains attached to the original stem. In like manner tle 
bath-room sponge is the fibrous skeleton secreted by a colony of 
sponge polyps, each of which is of extremely simple structure, though 
it differs in some particulars from the coral polyp. The students should 
examine and sketch pieces of coral and sponge so as to understand the 
process by which these substances have been built up by the polyps. 
A typical instance of a Celenterate animal is Hydra. This isa 
little jelly-like creature which is to be found in tanks. It has a bag- 
shaped body, with a series of hollow tentacles around the mouth. It 
is usually to be found attached to water plants by its basal portion, 
The body wall is composed of two layers of cells, The inver layer consists 
of large cells, many of them furnished with cilia, which serve to propel 
the contents of the digestive cavity. These cells also contain chlorophyll 
particles. The outer layer consists of small cells, amongst which are a 
number of curious explosive eells (thread cells) which assist in seizing 
