648 
PROTOZOA. 
THE ASTASID^. 
These are distinguished by an extremely contractile body, generally of a green or red color. 
The Euglena viridis (see page 646) is very contractile and assumes various forms ; it often ap- 
pears in the water m such myriads as to make it appear of a green color. 
THE PERIDINID^. 
In this family the species are provided with a silicious case or carapace, furnished with an 
opening which has a circlet of cilia: the shell is often produced into curious horn-like processes. 
Motion in these animals is not only effected by the cilia, but also by the aid of filiform append- 
ages protruded from the carapace. 
THE OPALINID^. 
These animals are colorless, of a glassy transparency, moving by cilia arranged in oblique lines, 
upon their fiat, oval bodies; they have only been found as parasites in the intestines of frogs and 
certain worms. 
Clsiss II. POMIFEMA. 
This term is from the Latin porus, pore, and fero, to bear, and the animals to which it refers 
are popularly called Sponges. These 
are generally regarded, and perhaps 
justly, as standing on a sort of debat- 
able ground between the animal and 
vegetable kingdoms, or at all events as 
occupying a frontier station in the 
former and approaching more closely 
to plants than any other animated be- 
ings. Sponge, in the state in which 
we usually see it, consists of a congeries 
of horny filaments, interlaced in every 
direction so as to form an intricate 
net-work of intercommunicating cells. 
Imbedded in these threads, in the ma- 
jority of sponges, are a number of very 
minute needle-pointed siliceous or cal- 
careous particles of various forms ; 
these are called spictda. In most cases, 
the spicula are simply of an acicular 
form, slender and cylindrical, and 
pointed at both ends. In other instances they have a small knob at one end, whUe the 
opposite extremity is pointed, giving them exactly the appearance of minute pins ; in others, 
again, we find one end transformed into a fork with two or even three prongs; or the whole 
spiculum consists of three or four spines of equal length. This framework, with its contained 
spicula, is, however, only a sort of skeleton, on which the true living portion of the sponge is 
supported. This consists of a coating of gelatinons matter, which is spread over all the fibers of 
the reticulated skeleton ; its consistence is very like the white of an egg, and it runs freely away 
from the sponge when the latter is taken out of the water. But when examined under the 
microscope, this gelatinous coating is found to consist entirely of an immense number of aggre- 
gated sarcode-cells, exactly resembling the Amoeba, the simplest type of the Rhizopoda, which 
we shall hereafter describe. Like that curious creature, each of these cells appears to possess 
a perfectly independent. existence ; each presents one or more contractile spaces; and even 
SPONGES OF VARIODS FORMS 
