SUBKINGDOM 



ARTICULATA (Annulosa). 



The Artieulata — incomparably the most numerous division of 

 the Animal Kingdom — include all the invertebrates having jointed 

 bodies. The bod}', more or less elongated and bilaterally sym- 

 metrical, is usually divided into segments or articulations. The 

 appendages, jaws, limbs, &c. are arranged bilaterally. The ner- 

 vous system consists of one or two ganglia in the anterior part 

 of the body, or of a double chain of ganglia in the ventral side. 

 The skeleton is external, the skin hardened in most cases into a 

 chitinous crust, affording the necessary fulcra to the muscles of 

 locomotion. This derma-skeleton is typically in the form of 

 successive rings, which, in the higher orders, are soldered to- 

 gether, so that only two or three divisions, or perhaps one solid 

 shield, can be discovered. 



This very comprehensive subkingdom may be divided as fol- 

 low: I. Arthropoda (including the classes Insecta, Myriapoda, 

 Arachnida, Crustacea). II. Anarthropoda (class Annelida). 

 III. Scolecida (class Annuloida). Some authors separate the first 

 division far from the other two, which are placed in relationship 

 with the lower mollusks. 



The palgeontological history of the artieulata is much behind 

 that of the other divisions of the antmal kingdom. So uni- 

 versally distributed and numerically abundant at the present day, 

 they are least perfectly represented among the relics of the past. 

 Their manifold organization, which in the recent state fits them 

 so admirably for generic and specific comparisons, is fatal to their 

 entire preservation, and the fossil examples are often so frag- 

 mentary as to admit of little more than the determination of 

 their class and family. The number discovered forms but a 

 small proportion of those which have probably existed. The 

 oldest fossiliferous rocks (Cambrian) bear many tracks and bur- 

 rows of marine worms, and a variety of specialized Crustacea 

 (Trilobites). Air-breathing articulates doubtless came into being 

 along with land plants. Insects being found in the Devonian. 

 Representatives of each class are found in the Palaeozoic rocks. 

 In fact, every main type of invertebrate animal is present in the 

 Lower Silurian. 



