INTRODUCTION. 
21 
SPIDKKS : DIVISION AKTICULATA. 
certainty. The sensft of toucli of course resides in the general integument ; but special organs of 
touch are also frequently developed. 
The mouth is nearly always furnished with several pairs of jaws, placed one behind the other, 
some serving for the prehension and others for the mastication of food. These jaws open later- 
ally, so that the aperture of the mouth is vertical, or in the direction of the axis of the body. 
Most of the Articulata have whitish or colorless blood. The only exceptions are to be met with 
among the worms, some of which have red blood. In these, however, the color of the blood is 
inherent in the fluid portion, and not due to the presence of red corpuscles. Their circulation 
is effected by means of a dorsal vessel, which carries the blood from behind forward : it returns 
to the posterior portion of the body, either through a proper vascular system, or by passing 
through interstices left in the tissues of the body. 
Sexual oviparous reproduction prevails throughout this division. The sexes are generally sep- 
arate, although in some of the lower forms we meet with complete hermaphrodism. 
In the fourth great division of animals, the bilateral type of structure is far from being so 
distinct as in the Articulata. It is still, however, to be recognized in the general arrangement of 
the external organs, especially of those surrounding the head. 
Those animals of which the snail, clam, oyster, and nautilus may be taken as familiar examples, 
are usually inclosed in a tough skin, to the inner surface of which the muscles are attached, and 
by its contraction and dilatation the movements of the animal are effected. With the exception 
of the cuttle-fishes, in which a sort of cartilaginous support is present, none of these creatures 
possess any thing which can be regarded as analogous to a skeleton ; the body forms a soft mass, 
frequently varying greatly in form at the will of the creature. These peculiarities have led zoolo- 
gists to give them the name of molluscous or soft-bodied animals : they constitute the division 
MOLLUSCA. 
In most of these animals the nervous system consists of a number of knots or ganglia, scattered 
more or less irregularly through the body, united with each other by nervous filaments, and giving 
off finer filaments, the true nerves, to the various organs. In the more highly organized Mollusca, 
three or four of these ganglia are collected in the head, forming a cephalic mass, which represents 
a brain ; but even in its most condensed form, the cephalic ganglia may still be recognized, form- 
ing a sort of ring through which the oesophagus passes. 
