GENERAL CHARACTERS OF ARTICULATA. 41 



the purpose of the latter, are discharged together. Although very voracious, 

 when supplies of food come in their way, most of the Mollusca are capable 

 of fasting for long intervals, where none offer themselves, a fact which is 

 readily explained by that general inertness of their vital processes, which has 

 been stated to be the characteristic of the group. 



General characters of Articulata. 



27. The members of the sub-kingdom ARTICULATA are distinguished for 

 the most part, by characters which are exactly opposed tfj those just enume- 

 rated. Their characteristic form is easily defined; and in no instance is there 

 any wide departure from it. The body is more or less elongated, and pre- 

 sents throughout a most exact bi-lateral symmetry. It is completely enclosed 

 in an integument of greater density than the rest of the structure, which is 

 divided into distinct rings or segments ; these, being held together by a flexi- 

 ble membrane, allow considerable freedom of motion, whilst they firmly pro- 

 tect the soft parts, and afford attachment to numerous muscles. It is in the 

 Centipede, arid other such animals, that this division into segments is most 

 distinctly and regularly marked. In the lower Articulata, such as the Leech 

 and the Earth-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 Crustacea and Arachmda, the segments are so closely united together, 

 as to be in some instances scarcely recognizable. In the former, the move- 

 ments 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 Articulata 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 complexity. 



28. 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 muscles with the aliment necessary to main- 

 tain their vigour. The power of these muscles is so great in proportion to 

 their size, that in energy and rapidity of movement, some of the Articulated 

 tribes surpass all other animals. 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 in- 

 stances 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 propensi- 

 ties, which are adapted to meet every ordinary contingency, being of similar 

 character in each individual of the same species, and presenting but little ap- 

 pearance 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.* 



29. All the Articulata, save a few of the very lowest species, possess a dis- 

 tinct head at one end of the body, furnished with organs of special sensation, 



* Reference is here especially made to the celebrated problem of Miraldi ( 155). 



4* 



