468 



CHAPTER XXVII. 



SUB-KINGDOM V.—ANNULOSA. 



Characters and Divisions of Annulosa — The Scolecida 

 and anarthropoda. 



Sub-kingdom Annulosa. — The Annulose animals are characterised 

 by the possession of a body which is usually more or less elongated, 

 and is always bilaterally symmetrical instead of being radially dis- 

 posed. Very commonly the body is divided into similar (homonomous) 

 segments, which may be definite or indefinite, and are arranged along 

 an antero-posterior axis. Lateral appendages may be absent or 

 present, and when present, are symmetrically disposed. A nervous 

 system is present, and consists of one or two ganglia placed in the 

 anterior part of the body, or of a ventrally -placed, double gangliated 

 chain. 



The sub-kingdom of the Annulosa may be divided into the three 

 primary sections of the Scolecida, the Anarthropoda, and the Arthro- 

 poda, of which the two former are often separated to form a single 

 great division of the animal kingdom under the name of Vermes. 

 The division of the Arthropoda comprises the Crustaceans, the 

 Arachnids, the Myriopods, and the Insects, the great majority of 

 which are provided with a resisting exoskeleton, which is typically 

 composed of chitine, but is often more or less largely calcified. 

 Hence, the record of this division in past time is a comparatively 

 complete one, its earliest representatives appearing in deposits of 

 Cambrian age. The division of the Anarthropoda comprises the 

 typical " Worms " (Leeches, Earthworms, Sea- worms, &c), in 

 which exoskeletal structures are often completely wanting, or are 

 only partially developed. Hence the palaeontological history of 

 these animals is a necessarily very imperfect one. We have, how- 

 ever, unquestionable proofs of the existence of Sea-worms in rocks 

 as ancient as the Ordovician, at any rate ; and, if we may judge 

 from the evidence of " tracks " or " burrows," the lower Cambrian 



