387 



THIRD GREAT DIVISION OF THE ANIMAL KINGDOIM 



THE ARTICULATED ANIMALS. 



This third general type of organization is quite as strongljf characterized as tj^at 

 of the Vcrteh.ata. The skeleton is not internal, as in the latter: but is seldom 

 altogether absent, as in the IVIolluslvS. The articulated rings wliich encircle the brdv, 

 and frequently the limbs, supply the place of skeleton — and being, in almost eveiy 

 instance, tolerably hard, furnish the necessary resisting fulcra to the muscles of loco- 

 motion ; whence, as among the Vertebrates, we find that the several actions of stepping, 

 running, leaping, swimming, and flying, are performed by them. Tlicre are also some 

 families among them that arc either footless, or have merely soft and membranous 

 articulated limbs, by wlrich they can at most crawl. This external position of their 

 hard parts, with the muscles inward, reduces eacli articulation to the condition of a 

 case, and only permits of two kinds of movements. When attached to the next arti- 

 culation by a closed joint, as in the instance of the limbs, the only motion is by 

 ginglymus, that is, in a single direction, so that numerous articulations are required to 

 impart variety of action ; and from this results a very great loss of power in the 

 muscles, and consequently a general feebleness in the creature in proportion to its 

 magnitude. The articulated pieces which compose the body frame-work, however, 

 are not alwavs thus connected ; being oftener united by flexible membranes only, 

 wliich slide considerably one over another, and so allow of more varied movements, 

 but not of tlie same force. 



The system of organs in which all Articulated Animals bear the nearest resemblance 

 to each other, is that of the nerves. 



Their brain, placed over the oesophagus, and supplying nerves to the jiarts ad- 

 jacent to the head, is very small. Two chords, which encircle the oesophagus, are 

 continued along the abdomen, and are connected at intervals by double knots or 

 ganglia, from which the nerves of the body and of the limbs are sent forth. Each of 

 tiiese ganglia seems to perform the functions of a brain to the adjoining parts, and 

 continues for a certain time to confer sensibility on them, al'ter the animal has been 

 divided. If to this lie added, that the jaws of these animals, whenever they have 

 any, are invariably lateral, and open and shut outward and inward, and not upwards 

 and downwards, and that in none of them has a distinct organ of smell yet been dis- 

 covered, nearly all has been expressed which it seems can be stated of them generally: 

 for the existence of organs of hearing ; the presence, number, and form of those of 

 sight; the productiveness and mode of generation*; their kind of respiration ; the ex- 



• A renmrkuble discoverj- connectci with tl.is subject is that of j Srx- liis Uiiv(Tt;itiiii, nn the Ki:ff5 of Spiriers, M:iibmirg, 1324 ; and 

 M.Hcrolii, who found that in the c^g of Crustaceans and Arach- that of .■\1. Rathlte uu the Eij^^s of Crabs, Lejpsic, 1821. 

 iri'tes tile yyllt communicntes with the baeli tlircufh the interior.— 1 



