246 ARTICULATION. 



connexion with the superior powers of loco- of gills, and have no stigmata, or spiracles on 



motion afforded by these appendages, the sexes the surface of the skin. 



are separate, and the organs of vision are In the Articulate sub-kingdom, as in the ver- 



well developed, and often highly complicated, tebrate, there may be traced one general plan 



With the exception of some genera, as the of structure pervading all the classes, but with 



myriapoda, in which the body is divided into such variations in it as are, in each case, de- 



a number of nearly equal segments, and of manded by the particular exigencies of the 



the arachnida and many Crustacea, in which individual to which it is applied; but these 



the head and thorax are blended together, the variations are of such a nature, that a gradation 



body of the condylopes of Latreille is divided of complexity or perfection may be followed 



into three principal parts, viz., the head, which through all the organic systems. With regard 



bears the antennae, the eyes, and the mouth ; to locomotion, we commence with a class (the 



the thorax, which supports the feet and the Cirripeds) as fixed and immoveable as the 



wings, when the latter are present; and the polypes and sponges of the Acrite sub-king- 



abdonien, which contains the principal viscera. dom ; and afterwards trace a series of forms 



These segments present different degrees of adapted first to slow and tortuous reptation ; 

 hardness in the different classes of condylopes, next to swifter progression, as creeping, run- 

 being most flexible in the arachnidans, firmer ning, or leaping ; and, lastly, to a rapid flight 

 in the insects, and calcareous in most of the through aerial space. 



crustaceans. The origin of the insertions or Generation, in like manner, is effected, in the 



articulations of the body which form so marked lowest class, without the intercourse of separate 



an external character of these animals, is as individuals; afterwards by the reciprocal irn- 



follows : pregnation of co-equal hermaphrodites, and, 



The integument is composed of two layers or lastly, as in the vertebrate division, by indi- 



pellicles, viz., the epidermis and the corium, viduals of distinct sexes. 



and is originally of equable consistence, and The perfection of the nervous system results 



presents an uninterrupted continuity, save by from the approximation of many separate gan- 



some slight transverse superficial wrinkles, glions into fewer masses of nervous matter. The 



The epidermis subsequently becomes solidi- organs of the senses also augment in number 



fied, in arachnidans and insects, by the super- and complexity. 



addition of a peculiar substance termed chitine, The Articulata present, in the organs of the 

 and in crustaceans by a calcareous deposition, vital functions, as strongly marked differences 

 so as to be divided into bands or rings. As the as are met with in the vertebrate animals, 

 external development proceeds, these epidermic With respect to the sanguiferous system, a 

 pieces are detached posteriorly from the inferior gradation may be traced from a circulation 

 pellicle, or corium; and the intervals of the in closed vessels to a diffused condition of 

 segments remaining membranous, and preserv- the nutritious fluid ; and a corresponding pas- 

 ing their flexibility, yield readily to the various sage from the articulata which respire by 

 movements and inflections of the body. means of circumscribed branchiae, to those 



The 1 1 Id class of articulate animals or In- in which indefinitely ramified tracheae carry 



sects ( Insecta), are either myriapod or hexapod. the air to all the parts of the body. The 



Most of the latter are furnished with wings, which amount of respiration thus produced occasions 



they acquire at a certain age, after undergoing the same effects here, as in the Vertebrate 



metamorphoses varying in kind and degree. In sub-kingdom, and the Insects thus constitute, 



every state they respire by tracheae, or elastic as it were, the Birds of the Articulate division 



vessels which receive the air by stigmata, situ- of animals. 



ated along the sides of the body. A dorsal (Richard Owen.) 

 vessel propels the circulating fluid, which is 



afterwards diffused throughout the cellular ARTICULATION (in anatomy), synony- 



tissue of the body. They have conglomerate or ,mous with joint. (Gr. apOpov. Lat. articulus, 



compound eyes, and antennae. arthrosis, junctura. Fr. articulation. Germ. 



IV. The Arachnidans (Arachnida, Spiders, Articulation, Gelenk. Ital. articolo). 

 Scorpions, &c.), are octopod and apterous; The power of motion, to an extent however 

 they have no antennae, and have simple eyes, limited, seems to be inseparable from our idea 

 Their circulation is effected by a dorsal vasi- of an animal, and in looking through the animal 

 form heart which transmits arterial branches, series we find none which do not appear to be 

 and receives the returning blood from veins, endowed with this power whether for the pur- 

 Their organs of respiration vary, some pos- pose of progression, or simply of altering the po- 

 sessing true pulmonary sacs which open upon sition or condition of some part of their bodies 

 the sides of the abdomen, others receiving the with respect to the others. The organic structure 

 air by tracheae, like insects. In both cases, which is the immediate agent in this motive 

 however, the air is respired by lateral orifices power (the muscular fibre), is one and the same 

 or true stigmata. throughout the whole chain of animals, va- 



V. The Crustaceans (Crustacea) have never riously modified according to the degree and 

 less than ten feet; they have two compound force of the motions necessary for the particular 

 eyes, and also antennae, which are generally individual. The mechanism by which this 

 four in number; their blood, which is white, structure acts upon the different parts of the 

 is circulated by means of a muscular ventricle body varies considerably, and increases in 

 situated on the backv They respire by means complexity as the forms of the animals them- 



