GENERAL CHARACTERS OF ARTICULATA. 49 



worm, the integument is altogether so soft, that the intervals of the articulations 

 are not very distinct from the rings themselves ; and in the highest Crusta- 

 cea and Arachnida, the segments are so closely united together, as to be in 

 some instances scarcely recognizable. In the former, the movements of the 

 body are entirely effected by its own flexion ; whilst in the latter, they are 

 committed to members developed for that special purpose. These members 

 also have an articulated external skeleton. The bulk of the body in the Ar- 

 ticulata is made up of the muscles, by which the several segments and their 

 various appendages are put in motion ; these muscles have their fixed points 

 on the interior of the hard envelope, just as they are attached in Vertebrated 

 animals to the exterior of the bones ; and they form a system of great com- 

 plexity. 



17. The development of the organs of nutrition in Articulata, would seem 

 to be altogether subservient to that of the Locomotive apparatus, their func- 

 tion being chiefly to supply the nerves and muscles with the aliment necessary 

 to maintain their vigour. The power of the muscles is so great in proportion 

 to the size of the animals, that in energy and rapidity of movement, some of 

 the Articulated tribes surpass all other beings. Their movements are directed 

 by organs of sensation, which, although not developed on so high a plan as 

 those of some Mollusca, are evidently very acute in their powers. There are 

 very few instances of Articulated animals being in any way restrained as to 

 freedom of locomotion ; and these are found in a single group, the Cirrhopoda 

 or Barnacle tribe, which connects this sub-kingdom with the last. In general, 

 they roam freely abroad in search of food, and are supplied with prehensile 

 organs for capturing their prey, and with a complex masticating apparatus for 

 reducing it. Their actions are evidently directed almost solely by instinctive 

 propensities, which are adapted to meet every ordinary contingency, being of 

 similar character in each individual of the same species, and presenting but 

 little appearance of ever being modified by intelligence. Hence these animals 

 seem like machines, contrived to execute a certain set of operations ; many of 

 them producing immediate results, which even Man, by the highest efforts of 

 his reason, has found it difficult to attain. 



18. All the Articulata, save a few of the very lowest species, possess a 

 distinct head at one end of the body, furnished with organs of special sensa- 

 tion, and with lateral jaws for the prehension and reduction of food ; and their 

 movements, being principally guided by the special senses, take place in this 

 direction. The bi-lateral symmetry of the body is not confined to its exte- 

 rior ; for it prevails most completely in the whole muscular apparatus ; and 

 even the organs of nutrition present more distinct traces of it than are to be 

 seen elsewhere. The compact heart of the Mollusca, for instance, is here 

 replaced by a long tube, the dorsal vessel, placed on the median line ; and the 

 respiratory organs, which are usually diffused through the whole system, are 

 uniform on the two sides. Even the intestinal canal partakes of this symme- 

 try ; in some species it runs straight from end to end of the body ; and even 

 where it is otherwise disposed, its appendages are nearly equal on the two 

 sides. The respiration of this group is for the most part aerial ; and the ap- 

 paratus for the purpose consists of a series of chambers or tubes, which are 

 dispersed or extended through the whole body, and which are expanded at 

 certain points, in insects possessing considerable powers of flight, into large 

 air-sacs. By this means, the air, the blood, and the tissue to be nourished, 

 are all brought into contact at the same points ; and a much less vigorous cir- 

 culation is required than would otherwise be needed ; whilst, at the same 

 time, the specific gravity of the body is diminished, and flight thereby rendered 

 more easy. The whole apparatus of nutrition is comprised within a compa- 

 ratively small part of the body ; and the bulk of the organs which compose 



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