38 PALEONTOLOGY 



Province IL— AETICULATA. 



In the great division of invertebrate animals called Arti- 

 culata the brain is in the form of a ring encircling the gullet. 

 A double ganglion above the tube supplies the chief organs of 

 sense. The ganglions below the tube are connected with two 

 chords which extend along the ventral surface of the abdomen, 

 and are in most species united at certain distances by double 

 ganglions, which are connected with the nerves supplying the 

 body segments and their appendages. The body presents a 

 corresponding symmetrical form. The skeleton is external, and 

 consists of articulated segments of a more or less annular form. 

 The articulated limbs, in the species possessing them, have a 

 like condition of the hard parts, in the form of a sheath which 

 incloses the muscles. The jaws, when present, are lateral, and 

 move from side to side. 



The worm, the lobster, the scorpion, and the beetle, ex- 

 emplify this province. 



The articulate division of the animal kingdom, most uni- 

 versally distributed and numerically abundant at the present 

 day, is least perfectly represented amongst the relics of the 

 former world. Their chitinous integuments, often hardened 

 with earthy salts, are quite as capable of preservation as the 

 shells of the Mollusca, and remains of them are met with in all 

 aqueous deposits ; but that manifold, complex 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 fragmentary as to admit 

 of little more than the determination of their class and family. 



The most ancient fossiliferous rocks bear imprints which 

 have been regarded as the tracks and burrows of marine worms. 

 Willi these are found Crustacea of the Lowest division, and of 

 a group which is wholly extinct, A little later appear the 



