ARTICULATED ANIMALS 309 



the rest of the body) had the form of a continuous unsegmented rod, the 

 animal would either stride along as if raised on stilts, or be quite unable 

 to raise the body from the ground, like, e.g., the earth-worm. In either 

 case it could only move extremely slowly and awkwardly. This, how- 

 ever, would be out of the question in the case of- an animal which 

 usually has to seek food scattered over a vast area (consider, on the 

 other hand, the slow-moving larvas of lepidopterous insects and the 

 limbless larvae of flies, etc.). It is therefore imperative that the legs 

 should be capable of moving and raising the body with the greatest ease. 

 Accordingly (like those of mammals) they are jointed, i.e., their chitinous 

 skeleton is divided into several segments which, like those of the body, are 

 united by thin articular membranes (see illustration, p. 311). This char- 

 acter distinguishes the animals of this division from all other inverte- 

 brates, and they are accordingly denominated Arthropoda, or animals 

 with jointed feet, i.e., legs. 



The mouth parts also, as we may learn from the development of the 

 young in the egg, are nothing more than limbs, which have undergone 

 alteration and have become adapted for the prehension of food. They 

 are closely crowded round the aperture of the mouth, moving hori- 

 zontally against each other and exhibiting the most varying types of 

 structure, according to the different nature of the food of the animals 

 (see under the separate classes). 



The antenna also represent modified limbs. They carry the organs 

 of touch, smell, and partly those of hearing. 



The Articulata also differ essentially from the Vertebrata hitherto 

 considered in the structure and disposition of the internal organs. We 

 shall here confine ourselves to mentioning only tbe most important 

 features. 



3. Nervous System (see illustrations, pp. 307, 311). — This is always 

 situated on the ventral side of the body. It consists of a series of nervous 

 ganglia, corresponding in number to the annular body segments, and 

 united by two nerve threads or cords, the whole presenting an appearance 

 not unlike a rope ladder. From the ganglia nerve fibres pass off to 

 the different parts of the body. The most anterior of the ganglia, 

 which lies below the oesophagus, is united by two nerve cords, which 

 embrace the alimentary canal, with a similar ganglion placed above 

 the oesophagus (inferior and superior oesophageal ganglia). The latter 

 is usually of considerable size, and sends nerves to the most important 

 of the organs of sense — the eyes and antennae. It is therefore considered 

 as the seat of the intellectual functions of the animal, and described as 

 the brain. 



4. Circulation. — In the Arthropoda the heart, as a rule, has the form 



21 — 2 



