ISO ZOOLOGY 



neighbourhood of the reproductive segments are modified and 

 assist in copulation (Fig. 91). 



Certain of the aquatic Oligochaets multiply asexually by 

 fission. In Aeolosoma, in many respects the most primitive of 

 the Oligochaets, one of the segments enlarges, forms a pro- 

 stomium, and then breaks off from the anterior half. In the 

 Naididae and the Chaetogasteidae, a "zone of fission " is formed 

 between two segments when the worm has reached a certain 

 size. This zone divides into two halves ; the posterior of these 

 forms a head for the posterior set of segments, the anterior 

 gives rise to a series of new segments forming the tail of the 

 anterior animal. In this way chains of zooids are formed. 

 These at length are set free, and differ from the mature 

 worm only in the absence of the reproductive organs, clitellum, 

 and genital setae, which they acquire later. 



No asexual reproduction is known amongst the Lumbrico- 

 morpha, but they possess a considerable power of reproducing 

 lost parts. 



THE POLYCHAETA. 



Characteristics. — Mariim worms, with numerous setae in bundles 

 borne on parapodia. The head is distinct, and usually bears 

 tentacles and palps ; the somites of the trunk carry cirrhi and 

 sometimes branchiae. As a rule the Polychaeta are dioecious 

 and have an indirect metamorphosis. 



The Polychaeta are divided in (i.) the Errantia and (ii.) 

 the Sedentaria or Tubicola ; these subdivisions are characterised 

 as follows : 



(i.) The Errantia are free and carnivorous, with a large pv- 

 stomium, which usually bears tentacles and eyes. The body is 

 rarely divided into regions ; the parapodia are large ; the pharynx 

 is protrusible and provided with chitinous jaivs or with papillae. 



Aphroditidae, Eunicidae, Nereidae, Syllidae, Alciopidae, 

 tomopteridae. 



(ii.) The Sedentaria are tube-building worms, whose tube inay 

 be fixed. Body often divided into regions. Prostomium and para- 

 podia small ; pharynx never armed with teeth; vegetable feeders. 



