388 ARTICULATED ANIMALS. 



istence of organs of circulation, and even the colour of the blood, olFer 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-blooded 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 diifers from the 

 rest, except by the presence of the mouth and of the principal organs of sense. Several 

 have their branchiae 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 antennse or articulated filaments, attached to the fore- 

 part of the head, and which are generally four in number ; besides which, they have 

 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 

 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 antennae. 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 



