246 



ARTICULATION. 



connexion with the superior powers of loco- 

 motion afforded by these appendages, the sexes 

 are separate, and the organs of vision are 

 well developed, and often highly complicated. 

 With the exception of some genera, as the 

 myriapoda, in which the body is divided into 

 a number of nearly equal segments, and of 

 the arachnida and many Crustacea, in which 

 the head and thorax are blended together, the 

 body of the condylopes of Latreille is divided 

 into three principal parts, viz., the head, which 

 bears the antennae, the eyes, and the mouth ; 

 the thorax, which supports the feet and the 

 wings, when the latter are present ; and the 

 abdomen, which contains the principal viscera. 



These segments present different degrees of 

 hardness in the different classes of condylopes, 

 being most flexible in the arachnidans, firmer 

 in the insects, and calcareous in most of the 

 crustaceans. The origin of the bisections or 

 articulations of the body which form so marked 

 an external character of these animals, is as 

 follows : 



The integument is composed of two layers or 

 pellicles, viz., the epidermis and the corium, 

 and is originally of equable consistence, and 

 presents an uninterrupted continuity, save by 

 some slight transverse superficial wrinkles. 

 The epidermis subsequently becomes solidi- 

 fied, in arachnidans and insects, by the super- 

 addition of a peculiar substance termed cliitine, 

 and in crustaceans by a calcareous deposition, 

 so as to be divided into bands or rings. As the 

 external development proceeds, these epidermic 

 pieces are detached posteriorly from the inferior 

 pellicle, or corium ; and the intervals of the 

 segments remaining membranous, and preserv- 

 ing their flexibility, yield readily to the various 

 movements and inflections of the body. 



The Hid class of articulate animals or JH- 

 sects ( Insecta), are either myriapod orhexapod. 

 Most of the latter are furnished with wings, which 

 they acquire at a certain age, after undergoing 

 metamorphoses varying in kind and degree. In 

 every state they respire by trachea, or elastic 

 vessels which receive the air by stigmata, situ- 

 ated along the sides of the body. A dorsal 

 vessel propels the circulating fluid, which is 

 afterwards diffused throughout the cellular 

 tissue of the body. They have conglomerate or 

 compound eyes, and antennae. 



IV. The Arachnidans ( Arachnidu, Spiders, 

 Scorpions, &c.), are octopod and apterous; 

 they have no antennae, and have simple eyes. 

 Their circulation is effected by a dorsal vasi- 

 form heart which transmits arterial branches, 

 and receives the returning blood from veins. 

 Their organs of respiration vary, some pos- 

 sessing true pulmonary sacs which open upon 

 the sides of the abdomen, others receiving the 

 air by trachea;, like insects. In both cases, 

 however, the air is respired by lateral orifices 

 or true stigmata. 



V. The Crustaceans (Crustacea) have never 

 less than ten feet; they have two compound 

 eyes, and also antennae, which are generally 

 four in number; their blood, which is white, 

 is circulated by means of a muscular ventricle 

 shunted on the back. They respire by means 



of gills, and have no stigmata, or spiracles on 

 the surface of the skin. 



In the Articulate sub-kingdom, as in the ver- 

 tebrate, there may be traced one general plan 

 of structure pervading all the classes, but with 

 such variations in it as are, in each case, de- 

 manded by the particular exigencies of the 

 individual to which it is applied; but these 

 variations are of such a nature, that a gradation 

 of complexity or perfection may be followed 

 through all the organic systems. With regard 

 to locomotion, we commence with a class (the 

 Cirripeds) as fixed and immoveable as the 

 polypes and sponges of the Acrite sub-king- 

 dom ; and afterwards trace a series of forms 

 adapted first to slow and tortuous reptation ; 

 next to swifter progression, as creeping, run- 

 ning, or leaping ; and, lastly, to a rapid flight 

 through aerial space. 



Generation, in like manner, is effected, in the 

 lowest class, without the intercourse of separate 

 individuals; afterwards by the reciprocal im- 

 pregnation of co-equal hermaphrodites, and, 

 lastly, as in the vertebrate division, by indi- 

 viduals of distinct sexes. 



The perfection of the nervous system results 

 from the approximation of many separate gan- 

 glions into fewer masses of nervous matter. The 

 organs of the senses also augment in number 

 and complexity. 



The Articulata present, in the organs of the 

 vital functions, as strongly marked differences 

 as are met with in the vertebrate animals. 

 With respect to the sanguiferous system, a 

 gradation may be traced from a circulation 

 in closed vessels to a diffused condition of 

 the nutritious fluid ; and a corresponding pas- 

 sage from the articulata which respire by 

 means of circumscribed branchiae, to those 

 in which indefinitely ramified tracheae carry 

 the air to all the parts of the body. The 

 amount of respiration thus produced occasions 

 the same effects here, as in the Vertebrate 

 sub-kingdom, and the Insects thus constitute, 

 as it were, the Birds of the Articulate division 

 of animals. 



(Richard Owen.) 



ARTICULATION (in anatomy), synony- 

 mous with joint. (Gr. otftopov. Lat. articulus, 

 artkrosis, junctura. Fr. articulation. Germ. 

 Articulation, Gelenk. Ital. articolo). 



The power of motion, to an extent however 

 limited, seems to be inseparable from our idea 

 of an animal, and in looking through the animal 

 series we find none which do not appear to be 

 endowed with this power whether for the pur- 

 pose of progression, or simply of altering the po- 

 sition or condition of some part of their bodies 

 with respect to the others. The organic structure 

 which is the immediate agent in this motive 

 power (the muscular fibre), is one and the same 

 throughout the whole chain of animals, va- 

 riously modified according to the degree and 

 force of the motions necessary for the particular 

 individual. The mechanism by which this 

 structure acts upon the different parts of the 

 body varies considerably, and increases in 

 complexity as the forms of the animals them- 



