Triploblastic Animals – Coelomates

Phylum Annelid the Segmented Worms

General Characteristics

Most of the worms with which we are familiar are included in this phylum. They are segmented and commonly called annelids. (From the Latin word for “little ring”)

The body is metamerically segmented. The body becomes divided transversely into a number of similar parts or segments. The subdivisions may be indicated externally by constrictions of the body surface. Internally, the segments are separated from each other by septa extending across the coelom. However, the various systems of body such as gut, blood vessels, and nerve cord are continuous throughout the length of body penetrating each individual segment.

The animals are triploblastic and coelomate, showing bilateral symmetry.

The annelids include worms, which may be marine (nereis), freshwater (stylaria) or found in damp soil (earthworms). Some are parasites, for example, Hirudo – leach).

The annelids show specialization of body structures. The organ systems are well developed.

Digestive system is in the form of alimentary canal which is divided into distinct parts, each performing a specific function. It has two openings, the mouth at the anterior end, and the anus at the posterior end. The mouth is overhung by a lobed structure, the Prostomium. In parasitic species, the digestive system is poorly developed. Annelids have true coelom i.e. the mesoderm splits into parietal layer which lines the body wall, and the visceral layer wwhich covers the alimentary canal, the space between the two layers of mesoderm, is the coelom, and is filled in by coelomic fluid, which serves as hydrostatic skeleton also.

Excretion takes place by specialized structures called nephridia. These are ciliated organs present in each segment in the body cavity.

A well developed central nervous system is present in annelids. It comprises of a simple brain and a solid double, longitudinal, ventral nerve cord. Nerves arise in each segment from the nerve cord.

Annelids are the first group of invertebrates which have developed a closed circulatory system – a system in which a circulatory fluid called blood flows in a network of vessels known as blood vessels. It transports gases and nutrients.

The respiratory system is absent. The exchange of gases is by diffusion through the skin in to blood capillaries. The skin is kept moist by mucus, and coelomic fluid.

The body wall contains muscles which help in locomotion. The muscles are of two types:

  1. Circular Muscles: These are arranged along the circumference of the body.
  2. Longitudinal Muscles: These are arranged along the length of the body.

The locomotion is brought about by the interaction of muscles and hydrostatic skeleton. Contraction of circular muscle produces a pressure in the coelomic fluid that forces the body to elongate. Similarly contraction of longitudinal muscles produce a pressure in the coelomic fluid that would cause the body to widen. The organs of locomotion in annelids are chitinous chaetae or setae embedded in sacs (earthworm) or on parapodia present in the body wall (e.g., Nereis). Chaetae are absent in leech.

The common mode of reproduction is sexual. Most annelids (Earthworm, leech) are hermaphrodite. In some annelids (e.g., Nereis) the sexes are separate, the fertilization is external and a free swimming trochophore larva is produced during the life cycle.

animals belonging to phylum annelida

Fig. 10.7 Examples of animals belonging to phylum annelida.

Burrowing activity of Earthworms permits greater penetration of air into the soil, and improves drainage capacity of the soil. It also enables roots to grow downwards through the soil more easily. Mixing and churing of the soil is brought about when earth which contains inorganic particles is brought up to the surface from lower regions. Earthworm is perhaps most active segmented worm in churning the soil, therefore it is commonly termed as natural plough.

Phylum Annelida comprises:

  1. Class polychaeta
  2. Class Oligochaeta
  3. Class Hirudinea

Class Polychaeta

These have a distinct head region with eyes and structure known as palps and tentacles. Sexes are usually separate. The organs of locomotion are parapodia. They are mostly aquatic (marine). During development these give rise to a trochophore larva. Important examples are Nereis, Chaetopterus.

Class Oligochaeta

These animals have internal and external segmentation. Organs of locomotion are setae. Head region not prominent or distinct. They are hermaphrodite (bisexual). No larva formed during development e.g. lumbricus terrestris, pheretima posthuma and other earthworms. They may be terrestrial or aquatic.

Class Hirudinea

They have body with fixed number of segments. Each segment has additional circular rings or markings called annuli. They do not have organs of locomotion and move due to the contraction of their body and with the help of suckers. Mostly hermaphrodite and trochophore larva is formed during development. They are aquatic. No distinct head is present but leeches have chitinous jaws for making a puncture in the skin of the host. They also have an anticoagulant secretion which is passed into the wound to allow smooth flow of blood into its digestive system where it can be stored for a long time e.g. Hirudo medicinalis (medicinal leech).

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