440 ZOOLOGY. 



cliaetae on every segment and usually borne on segmental 

 appendages called parapodia. 



SUB-CLASS : POLYCHAETA. 



A typical Polychaete has already been described in 

 Nereis. The diagnostic characters are : Cliaetae numerous 

 and large, often compound as in Nereis, and usually 

 borne on the segmental appendages or parapodia ; sexes 

 separate, with gonads in many or all of the segments; 

 coelomic ducts usually united with the nephridia into a 

 single series of tubes which function as both excretory and 

 genital ducts, except in Nereidae, in which the genital cells 

 escape by dehiscence. They are all marine and show a 

 great variety of forms, the parapodia being in many 

 reduced or much modified, and branchiae may be present 

 in addition. 



SUB-CLASS : OLIGOCHAETA. 



In the Oligochaeta, of which the earthworm is an ex- 

 ample, the parapodia are rudimentary and represented 

 only by a few chaetae. They are hermaphrodite, with the 

 gonads restricted to a few segments and genital ducts dis- 

 tinct from the nephridia ; the clitellum, a band of thickened 

 epidermis which secretes the cocoon, is usually present. 

 The head has no tentacles or other appendages as in the 

 Polychaeta. They live in the soil or in fresh water. 



PHYLUM ABTHROPODA. 



Literally the term Arthropoda means animals with 

 jointed feet, i.e. limbs (Greek arthron, joint; pous, foot). 

 This might be considered equally applicable to the 

 Vertebrata, but the joints or segments of the Arthropoda 

 are segments of thicker, more rigid, often more calcined 

 cuticle connected by parts of the cuticle which are thinner 

 and more flexible. One pair of these limbs or appendages 

 primitively occurs on each segment or metamere of the 

 body. At least one pair of the anterior appendages at the 

 sides of the mouth is adapted for use as jaws; for this 

 reason the name GnaUwpoda is sometimes preferred. In 

 the perfection of their metamerism Arthropoda resemble 



