INVERTEBRATE ZOOLOGY - H. J. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

SECTION TRIPLOBLASTICA, OR BILATERIA

SUBSECTION SPIRALIA

PHYLUM ANNELIDA

Most Annelids (from Latin annulus - a ring) are free-living animals inhabiting marine environments, freshwaters, and soils. Only a minor fraction of species leads a parasitic lifestyle. Their body size ranges from a few millimeters to 3 m. Over 14,000 species are known.

Annelids are metameric animals (from Latin meta - repetition, mera - part). They possess a Skin-muscular sac and a secondary body cavity, or coelom. Unlike the primary body cavity, the coelom has its own walls formed by mesodermal epithelium (Fig. 164). The coelom Functions as a hydrostatic Skeleton supporting the musculature and facilitates nutrient transport. It temporarily accumulates Metabolic waste products and participates in osmoregulatory processes. Its walls produce Gonads, and the cavity serves as a site for gamete maturation. The excretory Organs are metanephridia of ectodermal origin.

Class="center">

Fig. 164. Diagram of annelid anatomy (adapted from Hadorn):

a - fragment of a longitudinal section; b - transverse section: 1 - gut; 2 - cuticle; 3 - epithelium; 4 - septum;

5 - coelomic epithelium; 6 - metanephridium; 7 - coelom; 8 - dorsal Blood vessel; 9 - skin-muscular sac;

10 - perivisceral sinus; 11 - gonad; 12 - circular musculature; 13 - longitudinal musculature;

14 - ventral blood vessel; 15 - ventral nerve cord

Most annelids have a closed Circulatory system, meaning that blood circulates entirely within vessels and does not enter the body cavity, with a capillary network bridging Arteries and Veins. Respiration in most annelids occurs through the entire body surface, although some possess specialized respiratory organs, such as branchial skin gills.

The Nervous system consists of a Brain—a paired supraesophageal ganglion connected by circumesophageal

commissures to a pair of closely apposed (or fused) ventral nerve cords featuring paired ganglia in each segment. This type of nervous system is known as the ventral nerve cord.

The majority of annelids are gonochoric (dioecious), though hermaphrodites also occur. Gonads develop within the coelomic walls. Fertilization is either external or internal. Development in marine species (Class Polychaeta) proceeds via metamorphosis, whereas in freshwater and terrestrial forms it is direct.

As previously mentioned, most annelids are characterized by metamerism—the repetition of individual body parts and organs. Each body segment contains similar structural elements: paired coelomic sacs, nephridia, gonads, ganglia, circular Blood Vessels, and so on. If all segments appear externally identical, the metamerism is termed homonomous; if segments in different body regions differ structurally, it is heteronomous. Heteronomy results from the specialization of particular body regions to perform specific functions.

The phylum comprises two subphyla.