Question

what is body segmentation? in what structures is segmentation represented in the bodies of annelids, arthropods and vertebrates?

Answer 1

Segmentation may be defined as the characteristics of some animal species (also plants). Segmentation refers to division of body into repetitive segments arranged on longitudinal axis.

Each segment represents body parts, like components of the circulatory system, digestive system, nervous system and the excretory system.

Segmentation is is also referred to as metamerism and the sements as metameres (somites in animals, phytomers in plants). If segmentation is identical it is called homonymous metamary (Annelids), if unidentical, it is called heteronomous metamery (Arthropods, vertebrates).

1. Segmentation in Annelids:

• Annelids are segmented lophotrochozoans, with coelom. Their distinguishing characteristics are:

a. Chaetae or setae (bundles of chitinous thin walled cylinders, held by sclerotinized proteins).

b. Segmentation or metamerism

The outer tube of Annelid body is made of outer muscle sheath, epidermis and cuticle.

The inner tube is the coelomic cavity, which is filled with fluid, with suspended organs and tissues. Even the coelom is segmented.

• The segmentation occurs in form of disc or septa, which rooks like rings (Latin: “Anneli” is ring).
• The rings are thin sheets of mesodermal tissues, referred to as peritoneum. The segments are identical.
• Each segment thus formed, are modular units, with internal organs.
• The upper part of the body or the anterior segments represent true segments, is termed the prostomium (pro is front, stomium is mouth).

Prostomium contains the mouth. Tissues called peristomium, surrounds the mouth. Prostomium also contains the brain and the nervous tissues, which are repeated in their segments.

• Reproductive organs are restricted to few segments.
• Appendages and nephridia are also repeated.
• Some organs form a continuity with other segments (like blood vessels, gut tubes).
• The posterior post-segmental region, is termed as pygidium (in Greek, means “little tail”). It contains the anus.
• Just at the beginning of the pygidium, is the growth zone for Annelids. Thus, prostomium forms the oldest part of the body.

2. Segmentation in Arthropods:

• The outer body covering of Arthropods comprises of exoskeleton, which undergo molting (shedding). Exoskeleton is made of chitin and is hard. It is important for waterproofing, protection, attachment of muscles.
• Both internal and external body is segmented.
• The body of the Arthropods is segmented, but some segments are fused.
• The process of fusion of body parts is called tagmosis, and fused or integrated segments are called the tagmata. Includes head, thorax and abdomen.
• The head and thorax may be fused to form cephalothorax. The brain and compound eyes are located.
• The appendages are distributed among the segments, pair of appendages in each segment.

• Appendages are jointed (arthropod is jointed feet), may be branched (biramous) or unbranched (uniramous).

• The internal cavity contains internal organs. The main cavity is called the hemocoel, filled with hemolymph, comprising the open circulatory system.
• The nervous system is ladder like, paired nerve cord and ganglia and runs through all the segments.
• The respiratory and excretory system varies in distribution as per species.

3. Segmentation in vertebrates:

• Chordates are characterized by the presence of notochord (vertebral column for vertebrates).
•   They are coelomates with hetero-metameric segmentation and a proper circulatory system.
• During development of the vertebrates, three different segmental systems are formed, which are:

a. Somites, b. rhombomeres, c. pharyngeal arches.

In each of these units, there are further iteration of parts along long axis is established.

• The body wall of the vertebrates contains segmental units, like the vertebrae, inter vertebral discs, ribs, ligaments, muscles, nerves and blood vessels.
• Vertebrates have segmented vertebral columns and brain.
• Somites are the first morphologically distinct segment units. They are formed in pairs on either side of the axial organs.
• During later stage of development, fusion of tissues to form the skeletal and muscular segments occur.
• The compartmentalization occurs with differentiation.