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Scientific classification Kingdom Phylum Class Order Family Genus Species Animalia Echinodermata Asteroidea Forcipulatida Asteriidae Asterias A. rubens
Habit and Habitat of Asterias • Asterias is exclusively marine, bottom dwelling or benthonic animal. • Asterias is found equally abundant on hard, rocky, sandy or soft bottom. • The most species of Asterias are generally solitary but under certain ecological conditions, such as to avoid direct sunlight or excessive drying, many individuals may gather at some place for the purpose of protection.
Habit and Habitat of Asterias • All sea stars are carnivorous. • Many species of Asterias exhibit various types of biological relationships such as parasitism and commensalism, etc. ,
External Features of Asterias Shape, Size and Colour • Asterias has a radially symmetrical and pentamerous body. • The body consists of a central, pentagonal central disc from which radiate out five elongated, tapering, symmetrical spaced projections, the rays or arms. • The size varies from 10 -20 cm in diameter though some forms may be much smaller or longer. • The colour is variable having shades of yellow, orange, brown and purple.
External Features of Asterias Shape, Size and Colour • The body has two surfaces, the upper convex and much darker side is called the aboral or abactinal surface.
Starfish Dissection: Schematic View
Oral Surface The side of body, which in natural condition remains towards the substratum and contains the mouth or oral opening, is flat and of dark orange to purplish colour, is called oral or actinal surface. The oral surface bears the following structures, • Mouth • Ambulacral Grooves • Tube Feet or Podia • Ambulacral Spines • Sense Organs
Aboral Surface The side of the body, which remains directed upward or towards the upper surface, is convex and of light orange to purplish colour, is called aboral or abactinal surface. The aboral surface bears following structures, • Anus • Madreporite • Spines • Papulae or Gills • Pedicellariae
Body Wall of Asterias The body wall of Asterias consists of following tissue layers, • Cuticle • Epidermis • Nervous Layer • Basement Membrane • Dermis • Muscular Layer • Coelomic Epithelium
Alimentary Canal of Starfish In Asterias, the alimentary canal is tubular, straight, short and extends vertically along the oral-aboral axis in the central disc. It comprises the following parts, • Mouth • Oesophagus • Stomach • Intestine • Anus
Water Vascular System of Asterias The water vascular system is a modified part of coelom and it consists of a system of seawater filled canals having certain corpuscles. It plays most vital role in the locomotion of the animal and comprises madreporite, stone canal, ring canal, radial canal, Tiedeman’s bodies, lateral canals, and tube feet.
Locomotion of Asterias • Asterias lacks in head or anterior end, therefore, capable to move in any direction according to its desire. • It can move on horizontal as well as on vertical surfaces by the help of tube feet. Circulatory System of Asterias The so-called circulatory system includes following two systems: 1. Perihaemal system 2. Haemal system
Excretory System of Asterias • Asterias lacks well specialised excretory organs. • The nitrogenous metabolic excretory waste usually contains ammonium compounds. • The coelomocytes have significant role in the excretion of excretory wastes from the coelom.
Sense Organs of Asterias possesses a few primitive sense organs which are as follows • Eyes • Terminal Tentacles • Neurosensory Cells
EYES • The most significant sensory organs of Asterias. • Are simple, pigmented and occur at the base of terminal tentacles. • On the oral surface, at the base of each terminal tentacle occur optic cushions which are composed of the thick epidermis with many photoreceptors or pigmented cup ocelli
Asterias: Section through a terminal tentacle and eyespot
Terminal Tentacles • have sensory cells which are tactile and also sensitive to food and other chemical stimuli.
Neurosensory Cells • The entire body surface of Asterias is traversed by many neurosensory cells serving as chemoreceptors. • The neurosensory cells are slender cells with a fusiform body.
Reproductive System of Asterias • The reproductive organs of Asterias are of primitive type and lack copulatory organs, accessory glands, receptacles for storing ova and reservoirs for storing mature sperms. • There are only gonads which act as reproductive organs.
Life History and Development of Asterias
Life History and Development of Asterias Fertilisation • The most species of Asterias have only one breeding season in a year. • During breeding season, both types of mature sexes shed their sex cells in the sea and union of male and female sex cells or gametes (sperms and ova) occurs in sea water. • Fertilisation in Asterias is external.
Life History and Development of Asterias Embryogeny • The embryological development of Asterias is indirect and includes various larval stages. • The fertilised egg or zygote is spherical, half millimeter in diameter and contains little amount of yolk. • The cleavage is holoblastic and equal and it converts the unicellular zygote into a single layered, hollow, ciliated and spherical structure called coeloblastula.
Larval Development The larval development of Asterias includes the following larval stages • Bipinnaria Larva • Brachiolaria Larva
Metamorphosis • In about 6 or 7 weeks, the brachiolaria larva settles on the bottom or on some solid object and is fixed with that by its adhesive arms. • Now the bilaterally symmetrical larva metamorphoses into a radially symmetrical adult. • The larval mouth and anus close. • A new mouth is formed on the left side of the larva and a new anus is developed on the right side. • The newly detached rudiment of the body of sea star is less than 1 mm with short stubby arms.