ARTICULATED ANIMALS. 
388 
istence of organs of circulation, and even the colour of the blood, offer very great vari- 
eties,which must be studied under the various subdivisions. 
DISTRIBUTION OF ARTICULATED ANIMALS INTO FOUR CLASSES. 
The members of this great division, which have mutual relations as varied as they 
are numerous, still present themselves under four principal forms, whether we regard 
them externally or internally. 
The Annelides, Lamarck, or Red-Hooded Worms, constitute the first. In these, 
the blood is generally of a red colour, like that of the Vertebrates, and circulates in a 
double and close system of arteries and veins, which have sometimes one or several 
hearts or fleshy ventricles, tolerably well marked : they respire by organs, which are 
either developed externally, or are spread over the surface of the skin, or concealed 
internally. The body, which is more or less elongated, is always divided into nu- 
merous rings, of which the first, which is termed the head, scarcely differs from the 
rest, except by the presence of the mouth and of the principal organs of sense. Several 
have their branchice uniformly spread over the surface of the body throughout its 
whole length, or only aboujt the middle ; others, and such as inhabit tubes, generally 
have them only at the anterior portion. None have any articulated limbs ; but the ^ 
greater number are furnished with silky feet, or bundles of stiff and mobile filaments, 
instead of them. They are generally hermaphrodite, and some require a reciprocal 
fecundation. The organs of the mouth consist either of jaws more or less powerful, 
or of a simple tube : their external sensitive organs are fleshy tentacles, which in some 
are articulated ; and upon which are certain blackish points, that have been considered 
as eyes, but which are not present in all the species. 
The Crustaceans constitute the second form, or class, of Articulated Animals. These 
have articulated limbs, more or less complicated, attached to the sides of the body. 
Their blood is white, and circulates by means of a fleshy ventricle placed towards the 
back, which receives it from the gills, situate at the sides of the body, or at its hinder 
portion, and to which it returns by a ventral canal that is sometimes double. In the 
species last alluded to, the heart or dorsal ventricle is lengthened into a canal. These » 
animals are all furnished with antennae or articulated filaments, attached to the fore- i 
part of the head, and which are generally four in number ; besides which, they have -j 
several transverse jaws, and two compound eyes. It is among these only [through- | 
out the Articulata] that we find a distinct auditory apparatus. ; 
The third class of Articulated Animals is that of the Arachnides, which, in common j 
with a great number of Crustaceans, have the head and thorax joined into a single 
piece, with articulated limbs on each side, but the principal viscera of which are con- 
tained in the abdomen, which is attached to the hinder portion of the thorax. Their 
mouth is armed with jaws, and they have a variable number of simple eyes in the head ; 
but never any antennse. Their circulation is performed by a dorsal vessel, which 
gives out arterial ramifications, and receives venous ones ; but the manner of respira- ^ 
tion varies, some having true pulmonary organs with orifices leading to them at the 
sides of the abdomen, and others receiving air by means of tracheae, in the same 
manner as Insects. All, however, have lateral apertures for this purpose, or true ' 
stigmata. 
Insects constitute the fourth class of Articulated Animals, and the most numerous i 
