NORTH ZOOL. GAL.]] NATURAL HISTORY. 143 
(Cases 21 to 28,) are easily known from the other 
kinds by having only six or eight tentacles, furnished 
with one or more series of short processes on each 
side. Their stomach ends in six or eight elongated 
processes, which are considered as the oviducts. One 
genus of these animals ( Cornularia, Case 21) is said 
to have a horny external skin like the Tubularia , into 
which the animal is retracted for protection. There are 
some other genera which are said to have a hard carti¬ 
laginous external skin like the Zoanthi , as the Telesto . 
One genus has a similar tough skin, which is at length so 
hardened by the deposition of calcareous matter within 
its substance as to become a hard coral. This genus of 
animals live in groups side by side, and as they increase in 
size form shelly tubes, which has caused them to be called 
Organ Coral or Tubipora (Case 21). At certain intervals 
the animals contract themselves into their tubes, and 
bend out the soft part of the outer skin of the body near 
the head, so that it forms a broad collar round the end 
of the harder tube ; and as the different animals of the 
same group of tubes generally perform this action simul¬ 
taneously, the collars of the different tubes come in con¬ 
tact, and, having chalky matter deposited in them, unite 
and at length become stony like the rest of the tube. 
After this has been done, the animal begins to form 
another tube ; and after a time they repeat the same 
process again, so that the different tubes of the same 
group are united by cross layers into a single mass. The 
animal being bright green, and the coral vivid red, makes 
it a most beautiful object when alive. 
By far the greater number of the animals of this order 
have a very thick, spongy outer skin, which is often 
strengthened by having variously shaped calcareous 
grains, or rugose and more or less fusiform calcareous 
spiculae, imbedded in its surface. These animals live in 
societies closely united in a single mass by their outer 
skin, while there appears to be also a general community 
of function in their more important internal organs. 
They are constantly emitting buds from different parts 
of their surface, and as each species emits them in a pe¬ 
culiar form, the mass assumes a definite shape, varying in 
the different kinds. 
