INTRODUCTION. 
IT 
SPONGES AND COKALLINES : DIVISION PKOTOZOA. 
ANALYSIS OF THE SYSTEM OF CLASSIFICATION ADOPTED IN THIS V70EK. 
In this volume we shall adopt a system of classification suited to the present improved state of 
zoological science. It may be well to indicate, at this point, the mode of analysis by which this 
arrangement is reached. 
But it is necessary first to state that modern investigations, aided by the wonderful powers of 
the microscope, have "enabled scientific men to analyze the various substances of which the bodies 
of animals are composed, and to reduce them into their elements. Numerous and varied as are 
these substances — bone, cartilage, sinew, nerve, muscle, hair, teeth, nails, claws, even the trans- 
parent lens of the eye — all are reducible to one kind of structure, and this is a cell. All organic 
substances are made up of cells. The primary organic cell is a minute pellucid globule, invisible 
to the naked eye, and containing within it a smaller cell, called the nucleus^ which again contains 
a still more minute granule, called the nucledus, or little nucleus. Even the highest animals, in 
the early development of the embryo, are composed entirely of nucleated cells, which afterward 
assume the forms peculiar to the various tissues of which their bodies are comj)Osed, 
At the lowest point of the animal kingdom, verging so closely on the lowest forms of plants as 
to leave us at first in doubt to which of the great divisions of organized nature they should be 
referred, we meet with a series of creatures in which the functions of organic life are performed 
by its simplest element — the cell. From this circumstance they have received from naturalists 
the denomination of unicellular animals, or Protozoa. 
These animals, though presented to us in a variety of forms, from the simple monad up to 
the complicated sponges, consist entirely of elementary nucleated cells, or of aggregations of such 
cells, in which each still retains, to a certain extent, an existence independent of its fellows, and 
generally possesses the power, when separated from its attachments, not only of continuing its 
own life, but even of producing another compound structure similar to that from which it had 
been detached. These simple creatures possess no digestive cavity, their food, when solid, being 
received into the substance of the body, and there gradually assimilated. The nervous and vas- 
cular systems are equally deficient ; in fact, the nucleus, Avhich is an essential portion of the 
elementary cell, and one or more contractile vesicular spaces, are the only traces of internal organ- 
ization observable in the clear gelatinous substance of which they are composed. 
Reproduction is effected in general by the division of the substance of the animal : the phenom- 
ena of sexuality, which we shall meet with in all the higher animals, are here never witnessed. 
From these simple creatures we pass to a group of animals, the lowest members of which exhibit 
but little, if any, advance in point of organization. They do not, it is true, consist of isolated cells, 
"Vol. I.— 3 
