150 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 CHAETOGASTRIDAE, 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.—Marine 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 pro- 
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 ehitinous jaws or with papillae. 
APHRODITIDAE, EUNICIDAE, NEREIDAE, SYLLIDAE, ALCIOPIDAE, 
TOMOPTERIDAE. 
(ii.) The Sedentaria are tube-building worms, whose. tube may 
be fired. Body often divided into regions. Prostomium and para- 
podia small ; pharynx never armed with teeth ; vegetable feeders, 
