xii ARTHROPODA 155 



is merely a horny transparent skin, the moult still occurs at 

 intervals ; and it is probable that the reason for this is not 

 merely the need of rendering possible the growth of the 

 body, but also that the process is physiological, being the 

 means by which certain waste products of the body are 

 eliminated. 1 



General ^ ne Arthropoda form a very large group, all 



Arthropod alike in their bilateral symmetry and in having a 

 Character- segmented, chitinous, and calcified exo-skeleton 

 istics. enc iosing the soft boneless body. They all show 

 a more or less marked segmentation of the body externally. 

 Each segment bears a pair of the typical jointed legs which 

 give the name to the group (arthros, a joint ; pous, a leg). In 

 nearly all Arthropods the mouth is surrounded by a number 

 of curious little structures known as the mouth parts ; the 

 development of these shows that they are merely the ordinary 

 jointed limbs or appendages of the fused segments of that 

 region, modified for the purpose of catching food. Also on 

 the head there are two pairs of jointed feelers or antennae, 

 which again represent modified limbs. A body-cavity is 

 present, but this contains blood, being a development of the 

 vascular system and not a true coelom. 



Nervous The external segmentation of the body is 



System, correlated to some slight extent with a corre- 

 sponding repetition of the internal organs. This is shown 

 in the main nervous system, which consists of a series of 

 double ganglia, corresponding with the segments of the body 

 and connected by a double nerve cord. This cord with its 

 ganglia lies on the ventral side of the body, but in front it 

 divides and passes round the gullet to the dorsal side, and 

 there terminates in a specially well developed double ganglion, 

 from which nerves go to the eyes and antennae. This dorsal 

 ganglion constitutes the brain. 



There is a single heart, having a single cavity. 



of C Blood n ^ ^ es J us k a ^ove the alimentary canal, and by its 



pulsations it propels the blood throughout the 



body, but the circulation and aeration of the blood are slow, 



and its temperature therefore low, so that Arthropods are all 



"cold-blooded" creatures, i.e. their temperature varies with 



that of their environment. This fact explains the great 



1 Cambridge Natural History, vol. v. p. 169. 



