When we speak, however, of the occurrence of the Trochosphere throughout a wide range of animal life, it must be understood that its presence is not necessarily uniform throughout a group in which it occurs. Larval forms are adaptations which conform with the conditions of life for the particular animal in question: and nearly related kinds of animal may be without a larva. The Trochosphere larva is, of course, only adapted for aquatic existence, and is necessarily absent in the case of terrestrial forms.
Table of the Classification of the Animal Kingdom [A]
| Grade I.—Unicellular Animals. | PROTOZOA. | |||
| (Intermediate forms, see [p. 34].) | ||||
| Grade II.—Two-Layered Animals. |  | SPONGES. CŒLENTERATA. | ||
| Grade III.—Three-Layered Animals. | | PLATYHELMINTHES, or FLAT-WORMS. | ||
| Grade IV.—Cœlomata, or Three-Layered Animals with a Body-Cavity. |  | VERMES, the Higher Forms. ARTHROPODA. MOLLUSCA. BRACHIOPODA. BRYOZOA. ECHINODERMATA. TUNICATA OR ASCIDIANS. VERTEBRATA. |  | Chordata. |
[A] In the subsequent tables which show the respective sub-divisions of these chief groups, the larger only of the sub-divisions are named.
When an animal has no free larva, but quits the egg in a form practically identical with that of the adult, the development is said to be "direct." But changes equally startling with those displayed when a larva develops into the adult form, may take place while the young animal is enclosed within the egg itself. To these also zoologists apply the axiom referred to above, that the history of the individual summarises the history of the race. Thus, for example, the Amphibian larva, e.g. the tadpole of a frog ([p. 153]) has gills, which disappear in the adult form: the young reptile, bird, or mammal, which has no larval stage, has gills during a comparatively early stage; and loses them at a later period of its development. In each case zoologists conclude that the animal is descended from a fish-like ancestor, which possessed gills all its life, and that the more immediate ancestors in the family tree, have lost their gills by degrees.
The study of the progressive changes of young forms, whether larval, or enclosed within the egg, is called Embryology, and constitutes, in these days, the major branch of zoological science. That it is of paramount importance to the student of classification, engaged upon the sorting of the animal kingdom, will be apparent from what has been stated above.
CHAPTER IV
THE ONE-CELLED ANIMALS OR PROTOZOA
Some idea of the general characteristics of the Protozoa has already been given by the description of Amœba. We may now say something about special groups of the Protozoa, which have minor characteristics of their own.