CLASSES OF THE MOLLUSCA, 3 
Protozoa. In the same way we can pass from the highest of the 
Annulosa to the Protozoa. But we cannot find any continuous 
succession of adult forms which will connect the Annulosa with 
the Mollusca, or the Mollusca with the Vertebrata. 
Much use is made of the terms high and low in speaking of 
animals; and it is important to bear in mind that they are by 
no means intended to imply that there is any difference in the 
degree of perfection, or that one animal is less fitted to subserve 
the purposes of life than another. By an animal of a low 
organisation is simply meant one in which all the functions 
of life are carried on by means of a few organs. The greater 
the number of organs that are set apart to perform special 
functions the higher is the animal said to be. 
The evidence afforded by geological researches seems to show 
that the leading types of animal structure have existed from a 
comparatively early period in the history of the globe; and that 
all forms which haye left any indications of their existenco 
belong to one or other of these types. The oldest fossils known 
at the present time belong to the Protozoa; but next to them 
come the Mollusca. 
By adding to the living population of the world, those forms 
which peopled it in times long past, we may arrive at some 
dim conception of the great scheme of the animal kingdom. 
And if at present we see not the limits of the temple of nature, 
nor fully comprehend its design,—at least we can feel sure that 
there is a boundary to this present order of things; and that 
there has been a plan, such as we, from our mental constitu- 
tion, are able to appreciate, and to study with ever-increasing 
admiration. 
CLASSES OF THE MOLLUSOA. 
This sub-kingdom consists of two great groups, viz., tho 
mollusca proper and the molluscoida. The mollusca are animals 
with soft bodies, enveloped in a muscular skin, and usually 
protected by a univalve or bivalve shell. That part of their 
integument which contains the viscera and secretes the shell, is 
termed the mantle ; in the uniyalves it takes the form of a sac, 
with an opening in front, from which the head and locomotive 
organs project: in the bivalves it is divided into two lobes. 
The univalye mollusca are encephalous, or furnished with a 
distinct head; they have eyes and tentacula, and the mouth i: 
armed either with jaws or with tooth straps.* Cuvier has 
* One of the drawbacks to the study of mollusca is the prevalence of such teims a3 
jaws, arms, feet, &c. The reader must not suppose that the parts so designated are 
B2 
