The phylum Annelida contains segmented worms (such as the earthworm). The development of segmented bodies allowed the formation of specialised functions in different segments. Annelids have an enlarged coelom to accommodate more complex internal organs.
The well-developed, fluid-filled coelom and the tough integument act as a hydrostatic skeleton. There are about 12,000 marine, freshwater, and terrestrial species usually divided into three taxonomic classes. Similarities of larval forms to Mollusks suggest annelids share a common ancestral group.
Annelids have a closed circulatory system with blood vessels running the length of the body and branching into every segment. Closed circulatory systems are more efficient than open ones for moving materials within a body. The annelid nervous system consists of a brain connected to a ventral solid nerve cord, with a ganglion in each segment.
Annelids have a complete digestive system that include a pharynx, stomach, intestine, and accessory glands. Excretory nephridia in each segment collect waste material from coelom and excrete it through the body wall.
Classification of the Annelida:
i. Class Polychaeta:
Most annelids belonging to the taxonomic class Polychaeta, are marine and possess parapodia and setae. Parapodia are paddlelike appendages used in swimming that also serve as respiratory organs. Setae are bristles, attached to parapodia that help anchor polychaetes to their substratum and also help them move.
Clam worms, such as Nereis, are active predators. Many have well-developed cephalisation, with a head having well-developed jaws, eyes, and other sense organs. Sedentary filter feeders possess tentacles with cilia to create water currents and to select food particles. Only during breeding do polychaetes have reproductive organs. Polychaet zygotes develop into a type of larva similar to that produced by marine clams.
ii. Class Oligochaeta:
The class Oligochaeta includes earthworms that tend to have their few setae protruding in clusters directly from their body. Earthworms have poorly developed heads or parapodia. Locomotion is by coordinated movement of the body muscles and assistance of their setae. When longitudinal muscles contract, segments bulge and setae protrude and anchor into the soil. Circular muscles contract, causing the worm to lengthen, setae are withdrawn and the segment moves forward.
Earthworms reside in moist soil where a moist body wall facilitates gas exchange. Earthworms are scavengers that extract organic remains from the soil they eat. A muscular pharynx draws food into the mouth. Ingested food is stored in a crop and ground up in a muscular gizzard.
The dorsal surface of the intestine is expanded into a typhlosole that allows more surface area for digestion. External segments correspond to internal septa (walls) separating each body segment.
The earthworms’ excretory system has coiled nephridia tubules in each segment with two openings- one is a ciliated funnel that collects coelomic fluid, and the other is an exit in the body wall. Between the two openings, the coiled nephridia tubule allows removal of waste materials from blood vessels.
Red blood is moved anteriorly by a dorsal blood vessel and pumped by five pairs of hearts (sometimes referred to as aortic arches) to a ventral vessel. Earthworms are hermaphroditic, having both testes with seminal vesicles, and ovaries with seminal receptacles.
Mating involves the worms lying parallel to each other facing opposite directions and exchanging sperm. Each worm possesses a clitellum that then secretes mucus, protecting sperm and eggs from drying out. Embryonic development lacks a larval stage.
iii. Class Hirudinea:
The class Hirudinea includes leeches. Most are freshwater, but a few are marine or terrestrial. Each body ring has several transverse grooves. Leeches possess a small anterior sucker around the mouth and a larger posterior sucker. Although some are free-living predators, most are fluid feeders. Bloodsuckers keep blood from coagulating by hirudin, an anticoagulant in their saliva. Leeches were commonly used in early medicine to “bleed” the patient.