PHYLUM ANNELIDA 969 



seas. A few species live in brackish or fresh water. They are extremely 

 diverse in their habits. Some live in tubes and filter water for microscopic 

 food while others scrape up the thin film of organic debris that settles 

 on the bottom. Most members of the class are predaceous and have stout 

 jaws or denticles on an eversible pharynx that can be used to grasp prey. 



The sexes are separate and fertilization is external. Both eggs and 

 sperm are shed through tubules that connect each coelomic cavity with 

 the outside. In some species the segments producing gametes merely 

 burst to release them. Typically the eggs develop into planktonic larvae 

 called trochophores that swim about and feed, eventually metamorphos- 

 ing into worms and sinking to the bottom. In some, however, the eggs 

 are heavily yolked and hatch directly into small worms. 



O/igochaefes. Most of the fresh-water and terrestrial annelids be- 

 long to one of two other classes. Those that are wormlike and usually 

 lack eyes or appendages on the prostomium belong to the class Oligo- 

 chaeta (Fig. 15.1). Parapodia are also absent, but each segment bears 

 small tufts of a few chaetae. 



The oligochaetes include the large earthworms and smaller aquatic 

 worms. Earthworms burrow in soil or leaf-mold, eating their way 

 through the world, or they live in temporary burrows from which they 

 emerge at night to feed on the surface of the ground. Aquatic worms 

 burrow in mud or clamber on the vegetation, eating whatever debris 

 they can find. 



All oligochaetes are hermaphroditic. The testes are located in a few 

 anterior segments, with the ovaries in a few following segments. Pairs 

 copulate and the eggs are fertilized while they are on the outer surface 

 of the parent. Development is direct. 



Hirudinea. The other class of fresh-water and terrestrial annelids, 

 the Hirudinea (Fig. 15.1), includes the leeches or bloodsuckers, which 

 have one large sucker surrounding the mouth and another at the pos- 

 terior end of the body. Leeches share many characteristics with the 

 oligochaetes, especially in their reproductive systems, but have no ap- 

 pendages or chaetae. A few oligochaetes are ectoparasitic and have a 

 posterior sucker for attachment to the host. It is generally believed that 

 the leeches evolved from the oligochaetes through such transitional 

 forms. 



Archiannelida. A few marine annelids are very small and reduced 

 in their complexity, sometimes with no external segmentation, or no 

 chaetae, or with the body surface covered with cilia instead of a cuticle. 

 These were formerly thought to be primitive forms indicating that the 

 annelids evolved either from the flatworms or from trochophore-like 

 ancestors, and they were placed in a fourth class, the Archiannelida. 

 These worms are of particular interest as examples of simplification from 

 a more complex ancestor. Of the several genera the most markedly sim- 

 plified is Dinophilus (Fig. 15.2) which has only five or six segments and 

 no chaetae or parapodia. Its general structure resembles that of some 

 young polychaete larvae, and it is generally concluded that the archi- 

 annelids are "reduced" polychaetes. It is probable that the group in- 

 cludes genera that evolved independently from the polychaetes. At the 



