INVERTEBRATE ZOOLOGY IN THREE VOLUMES - BOOK 2 - H.Y. Shcherbak - 1996

PHYLUM ANNELIDA

Most Annelids are free-living animals inhabiting marine environments, freshwater bodies, and soils. Only a small fraction of their species leads a parasitic lifestyle. Their sizes range from a few millimeters to three meters. Over 12,000 species are known.

Annelids are highly evolved worms possessing all the Organ Systems characteristic of higher animal phyla: a secondary body cavity (coelom), a Circulatory system, locomotory Organs, and occasionally respiratory organs.

The body of an annelid consists of a HEAD lobe (prostomium), a trunk divided into ring-like segments, and an anal lobe (pygidium). They are characterized by the presence of locomotory organs—parapodia, which are mobile lateral outgrowths of the body bearing setae, sensory cirri, and sometimes gills. Parapodia are most fully developed in the Class Polychaeta.

Annelids possess a well-developed Skin-muscular sac covered externally by an elastic cuticle. In some groups, a ciliated epithelium is partially retained on the integument. The General structural plan of an annelid is shown in Fig. 1.

Fig. 1. Diagram of annelid anatomy:

a — longitudinal section; b — cross-section; 1 - gut; 2 - cuticle; 3 - epidermis; 4 - dissepiment; 5 - coelomic epithelium; 6 - metanephridium; 7 - coelom; 8 - dorsal Blood vessel; 9 - skin-muscular sac; 10 — circumintestinal sinus; 11 — gonad; 12 - circular musculature; 13 - longitudinal musculature; 14 - ventral blood vessel; 15 - ventral nerve cord

Unlike the primary body cavity, the coelom has its own walls formed by mesodermal epithelium. Typically, each trunk segment contains a pair (right and left) of coelomic sacs that line the inner surface of the skin-muscular sac and connect with each other above and below the gut. This creates an internal segmentation that corresponds to the external one. The coelomic sacs are filled with fluid, which constitutes the internal environment of the Organism. The coelom performs vital Functions: it acts as a hydrostatic Skeleton supporting the musculature; it facilitates nutrient transport; it can temporarily store Metabolic waste products; it participates in osmoregulation; and its walls bear Gonads while its cavity serves as a site for gamete maturation.

The Digestive System begins with the Mouth opening on the first trunk segment and consists of the Oral Cavity, Pharynx, midgut, and hindgut, the latter opening via the anus on the pygidium.

The excretory organs are tubular nephridia; generally, each segment contains a single pair of nephridia. There is an evolutionary transition from protonephridia, which are blindly ending at the inner tip, to metanephridia, which open into the coelom.

The circulatory system is typically well-developed and closed, meaning that blood moves within vessels and does not enter the body cavity.

The Structure of the circulatory system is uniform across all classes of annelids: dorsal and ventral vessels are connected by circular vessels; There is a circumintestinal blood sinus (or capillary network), along with capillary ramifications in the walls of the skin-muscular sac and gills, if present. Only in the class Hirudinea (leeches) does the circulatory system undergo reduction and replacement by a system of coelomic lacunae (see below).

Most annelids respire across their entire body surface, while some possess specialized respiratory organs—integumentary gills located on the parapodia or prostomium.

The Nervous system consists of a Brain—a paired suprapharyngeal ganglion—connected by circumoesophageal connectives to the ventral nerve cord. The latter consists of a pair of more or less closely appressed (or fused) ventral cords with paired ganglia in each segment and is termed the ventral nerve cord. Many annelids possess Sense Organs, including eyes, nuchal organs, and various tactile and chemosensory appendages.

Annelids include both gonochoristic (separate-sex) and hermaphroditic species. Gonads form in the walls of the coelom (with the exception of Representatives of the class Dinophilida). Fertilization is either external or internal. Cleavage is spiral and determinate. Development in marine species (class Polychaeta) involves metamorphosis, whereas freshwater and terrestrial forms exhibit direct development.

Most annelids are characterized by the repetition of certain body parts and organs. Each body segment contains homologous structural elements: paired coelomic sacs, nephridia, gonads, ganglia, circular vessels, and parapodia. This phenomenon of repeating similar structures is called metamerism. If all external segments are identical, the metamerism is considered homonomous; if segments from different body regions differ in structure, the metamerism is heteronomous. Heteronomy is the result of the specialization of specific trunk regions to perform particular functions.

The Classification of the phylum Annelida varies slightly among authors, but four classes are universally recognized: Polychaeta, Dinophilida, Oligochaeta, and Hirudinea. The first two classes are often united into the subclass/subphylum Aclitellata, and the remainder into Clitellata.