10 STRUCTURE OF ARTICULATA. 



skin which are harder and thicker than the rest of the body. 

 In some cases this annular arrangement arises solely from the 

 existence of a certain number of transverse folds or plaits which 

 groove the skin and encircle the body; but in most instances the 

 animal is enclosed in a species of solid armour, composed of a 

 series of rings united to each other in such a manner as to permit 

 of motion. The uses of this armour are similar to those of the 

 internal frame or skeleton of vertebrate animals ; because it de- 

 termines the general form of the body, protects the soft parts, 

 affords points of attachment for muscles, and furnishes them 

 levers, fitted to secure precision and rapidity of motion. It is 

 frequently termed an external skeleton, although it does not 

 represent our skeleton. In reality it is only the skin which 

 has become hard and stiff. Its rings are of a horny consist- 

 ence ; and in some instances, they become almost, if not 

 entirely, stony, forming a case in which the soft parts of the 

 animal are enclosed. 



3. In general, the rings of which this external skeleton is 

 formed are movable upon each other, but in certain parts of 

 the body, we sometimes see them soldered together, and then 

 they are less easily distinguishable : this is always the case in 

 the thorax of insects, but in other articulate animals, the cen- 

 tipedes or scolopendrse, for example, the rings are movable and 

 like each other throughout the whole length of the body. 



4. Some articulated animals have no extremities, an example 

 of which we have in the common leech ; but most of these ani- 

 mals are provided with them ; the number of these extremities is 

 very considerable; there are never less than three pairs, and 

 sometimes we find several hundred, as in some marine anneli- 

 dans. 



5. The nervous system of articulated animals is always com- 

 posed of a series of small ganglia attached together in pairs, 

 placed upon the middle line of the inferior face of the body, and 

 united by longitudinal cords of communication, so as to form a 

 sort of chain, or, rather, to represent a double-knotted cord, ex- 

 tending from one end of the body to the other. The nervous 

 mass formed by the first ganglion (Jig. 2, a), which is sometimes 

 called the brain, is enclosed in the head, and is placed above and 

 in front of the oesophagus ; the other ganglia, on the contrary, 

 are situate behind the oesophagus and beneath the digestive canal, 

 so that the cords which unite the ganglia of the head to those of 

 the thorax, pass from each side of the oesophagus and form 



3 Are all the rings of articulated animals movable ? 

 4. What is the number of extremities possessed by articulated animals ? 

 5 What is the character of the nervous system in articulated animals ? 

 Have these animals a brain, properly so called ? 



