114 MARINE INVERTEBRATES 
tinct divisions or chambers. The number of divisions varies 
with the different species and also with the age of the animal. 
Other partitions start from the outer sac, and extend toward the 
central axis, but do not unite with it. These partitions, called 
mesenteries, are always in definite multiples, varying in different 
species, new divisions growing between the first partitions in 
regular order. On the inner edge of these 
partitions the eggs of the animal are 
formed, which, when mature, drop into 
the chambers and pass through openings 
into the inner sae, or digestive cavity, and 
* out of the mouth into the water. 
The animals are classed according as the 
eggs are formed on all or on special par- 
titions, those being of the highest order 
Diagram of radiate structure. Where a limitation and constancy of func- 
tion is maintained. The upper surface 
of the body has hollow tentacles, each one of which opens into 
one of the chambers and extends outward. All parts of the 
animal communicate, and whatever enters the mouth circulates 
through the whole structure; and when assimilation is com- 
pleted the residue returns by the same road and is expelled 
through the mouth. This structure is common to all polyps; 
but there are great differences in their texture, some being soft 
and some horny, while others deposit a calcareous skeleton 
(corals). Some grow in colonies, like the hydroids and corals, 
and are stationary, others are free and independent; some have 
but few, others have many tentacles; and they differ widely in 
size, form, and color. 
Hydroids, sea-anemones, corals, sea-fans, starfishes, and sea- 
urchins are different examples of the radiate structure. They are 
not, however, all of them polyps. 
The Coelenterata are divided into four classes: Hydrozoa, which 
include the colonies of zodphytes which resemble seaweeds, the 
small jellyfishes which are born of these colonies, and the mille- 
pores, which are colonies of zodphytes which secrete a stony 
instead of a horny skeleton, yet differ in some respects from other 
