204 INVERTEBRATE MORPHOLOGY. 



the coeloni by a wide funnel-like extremity. Provisional 

 kidneys of the Turbellarian type occur in the larvae of many 

 Annelids, but the nephridia of the adult are, as a rule, of the 

 character just indicated and depart widely from the Turbel- 

 larian character. In the third place the reproductive organs 

 are developed in the peritoneal lining of the coelom and are 

 not usually (except in the Hirudiuea) provided with special 

 ducts. When mature the ova or spermatozoa are simply 

 shed into the ccelornic cavity and make their way to the 

 exterior through the ordinary nephridia, or through nephridia 

 specially modified for the purpose. Finally it may be mem- 

 tioned that a blood vascular system is usually present. 



I. CLASS Ghtetopoda. 



The Chaetopoda are Annelids in which the external seg- 

 mentation of the body corresponds with the internal seg- 

 mentation of the organs, and which bear along the sides of the 

 body two rows of pouches, the seta-sacs, the cells lining which 

 secrete chitinous spicules or setce of various shapes, which 

 serve for the purpose of locomotion or in some cases consti- 

 tute a defensive armament. 



The class is conveniently divisible into two subclasses. 



Subclass I. POLYCH2ETA. 



The forms included in this subclass are exclusively ma- 

 rine, and are characterized by the presence on the sides of a 

 greater or less number of the metameres of a pair of hollow 

 processes of the body-wall upon which the seta-sacs occur 

 and which are known as parapodia. In a few forms (Serpida) 

 the parapodia, and indeed the setae as well (Polygordius), may 

 be absent, and in others, such as Clymenella, they may be very 

 much reduced in size, but as a rule they possess a high de- 

 gree of development. In its typical form a parapodium 

 (Fig. 98) consists of a dorsal and a ventral lobe each of which 

 bears seta-sacs and setae (s). Towards the base of each lobe 

 there may frequently be found a slender hollow process, the 

 dorsal and ventral cirrus (dc and vc), and plate-like or more 

 or less dendritic appendages, the branchice (br), either modifi- 



