GENERAL DESCRIPTION 279 



In the unisexual annelids (most polychasts) the reproductive organs 

 are not well marked except during the period of breeding, when they 

 appear as specialized portions of the peritoneum. The hermaphroditic 

 annelids on the other hand have a complicated system of reproductive 

 organs (Fig. 479). The unisexual forms are mostly born in the form of 

 the trochophore larva, which goes through a complicated metamorphosis 

 before reaching the condition of the adult : in the hermaphroditic annelids, 

 the development is usually direct, the young worm when born having the 

 form of the parent. The body of the typical, primitive annelid may be 

 divided into two portions, the prosoma, or the primitive head, and the 

 metasoma, or the primitive segmented trunk. The trochophore larva, 

 which in most cases is supposed to represent the prosoma alone, is a 

 simple unsegmented animal, the metasoma not yet having made its appear- 

 ance. The metasoma soon begins to grow, however, at the posterior end 

 of the prosoma, the segments or somites developing one after the other as 

 the worm increases in length, until in some cases a hundred and fifty or 

 more may be present in the adult worm. In the higher annelids the 

 prosoma annexes one or more of the anterior somites of the trunk and 

 forms thus a head of increased size and complexity in which we can dis- 

 tinguish the two divisions already mentioned, the prostomium and the 

 peristomium. 



In the hermaphroditic annelids, which have a direct development, 

 these processes go on in the egg and the young worm is born with its 

 definitive form, although usually not with the full number of somites. 

 Many annelids reproduce also asexually by transverse divisions or by 

 serial or even lateral budding. 



Distributions and Habits. All annelids live in the water or in moist 

 places on the land or in the earth, the majority being marine. Most of 

 them are predaceous animals, although the oligochaets live chiefly on veg- 

 etable substances. The leeches are either predaceous or parasitic and the 

 Myzostomida are exclusively parasitic. 



History. It was Cuvier who, in 1798, first called attention to the 

 fundamental difference in structure between the higher and the lower 

 worms, and Lamarck who gave the former the name Annelides. Savigny 

 (1820) subdivided the group into the Annelides nereideae, serpuleae, lum- 

 bricineae, and hirudineae, and may be considered the founder of the 

 modern classification. Milne-Edwards (1834) introduced the subdivisions 

 Annelides errantes, tubicoles, and terricoles, which for sixty years or more 

 had a place in the system, and Grube (1851) the subdivisions Polychceta 

 and Oligoch&ta, which are still in general use. In more recent times 

 Ehlers has been perhaps the most active in the development of the system. 



The phylum contains about 4,500 species grouped in 4 classes. 



