ARTICULATES. 599 



body, there the segments are reduced in number and size, as in 

 the insects and the crabs ; in the Annelidans, as the Earth-worms 

 and Lug-worms, we find the number of segments increased. 



The animals of this class are active; hence, their skeletons 

 are light and thin. The muscles or organs of motion are at. 

 tached to the interior of the skeleton ; but as this is hard and 

 unyielding, it is necessary that it undergo a process of exuvia- 

 tion, which occurs in all the Articulate animals, going on through 

 all the stages of their existence. Phosphate of Lime, which 

 enters into the bones of the vertebrate classes, constitutes also 

 the material out of which the skeletons of a majority of these 

 animals are formed. Considering their size, it may be doubted 

 whether any other animals possess so large an amount of muscu- 

 lar power as the Articulates. The bulk of their bodies is really 

 made up, in great part, by the muscles which move them. 

 Throughout the animal kingdom, the muscular power corres- 

 ponds with the amount of respiratory action, and the development 

 of animal heat ; in various forms of the Articulates, this law is 

 remarkably displayed. 



The strongest resemblance to each other, exhibited by these 

 animals, exists in the nervous system. The brain is extremely 

 small. Two nervous cords, surrounding the oesophagus or gul- 

 let, run along the centre of the lower surface of the animal ; 

 these cords are studded, at regular intervals, with knots or 

 ganglia, forming so many centers from which the nerves pass off 

 to the different segments. 



The head also has its ganglia, in which the cords terminate 

 anteriorly. In cases where the members are not distributed 

 along the entire body, but limited to one part, as in Insects, 

 Arachnidans, and the higher Crustaceans, there is a corresponding 

 concentration of the ganglia in that particular part; indicating 

 by its degree, the elevation of the animal in the series. 



The organs of sense are very imperfectly developed, and in 

 some instances, entirely wanting, excepting that of sight. 



No organ of smell has been discovered, unless it be assigned 

 to the antennse. Some naturalists have described organs of 

 hearing in the insects, while others regard the antennse as instru- 

 ments for the exercise of that sense, and also of feeling. 



The digestive apparatus is, for the most part, in accordance 

 with the carnivorous habits of the Articulates. Where animal 

 food is eaten, the process of digestion is less complicated than 

 where vegetable food is used. 



The lengthened form of these animals impresses its character 

 upon their digestive, and also upon their circulating apparatus. 



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