139 * 
NORTH ZOOL. GAL.] NATURAL HISTORY. 
deposition of earthy particles into the older part of its 
substance. 
Thisdivision(LEioDERMATA)contains three classes; the 
Acalepha or Sea Nettles, the Zoophytes , and the Polyzoa. 
The first class of Acalepha, or Blubber-fishes , are so 
called from their being almost entirely formed of a soft, 
very cellular, jelly-like substance, which rapidly decom¬ 
poses after death, without leaving any hard part or ske¬ 
leton that can be preserved in a dry state ; they are des¬ 
titute of any true stomach, but only have vesicles in their 
substance. They float free on the ocean. A few animals, 
which appear to be intermediate between them and the Sea 
Anemonies, as the Porpita and Velella, have a cartilagi¬ 
nous plate sunk in their back, which keeps them of a de¬ 
fined form. The plate of the Porpita (Case 1) is circular 
and simple, while that of the Velella (Case 1) is oblong, 
with an erect crest placed obliquely across its back. 
Cases 1 to 30, are occupied with the hard parts of the 
second class, or Zoophytes, ( Zoophyta ,) which are so 
called because these animals are often crowded together 
in societies or families, in such a manner as to assume the 
form of trees. This form is taken to allow each of the 
individuals of which the society is composed to procure 
a fair and equal share of nourishment from the water 
which surrounds them, and on which they live, as the 
leaves of trees extract nourishment from the air in which 
the plant grows. 
Each individual animal of these zoophytes has a simple 
plaited membranaceous bag-like stomach, with only a 
single orifice, like that of the Star-fish, and their mouth is 
surrounded by a more or less numerous series of tentacles, 
which search for and conduct the food that comes within 
their reach into the digestive cavity, while the animal 
itself is generally attached by its back to marine bodies, 
and very commonly to others of its own kind ; the union 
which they form with each other is sometimes so intimate 
that one individual cannot be injured without its being 
felt by the whole society. The hard parts of these 
animals have been all classed under the name of Corals, 
but the animals which form them are of three very dif¬ 
ferent kinds, each forming a peculiar kind of skeleton for 
its protection. They are divided according to the form 
of the stomach, and the number of the tentacles, into 
