Internal Anatomy
Body wall and endoskeleton
The body wall of Linckia laevigata is approximately 3-4mm thick (Motokawa & Wainwright 1991). It consists of an upper epidermis layer, a middle thick, mutable, connective tissue dermis (or the catch tissue) and a coelomic epithelium (Ruppert, Fox & Barnes 2004). The catch tissue allows the sea star to alter its body wall’s stiffness and previous study has already proven that L. laevigata individuals would actively change their stiffness (Motokawa & Wainwright 1991). Sea stars have endoskeleton (skeleton inside body) and it is located inside the dermis layer (Ruppert, Fox & Barnes 2004).
Water Vascular System (WVS)
Water vascular system (WVS) is a highly derived coelomic cavity (i.e. body cavity) in the echinoderms. In particular, the water vascular system of a sea star consists of the tube feet and a series of internal coelomic fluid-filled vessels that runs through the central disc and the arms (Ruppert, Fox & Barnes 2004). As seen in the figure below, a circular ring canal is located inside the central disc of the sea star. The ring canal is then connected to the five radial canals that extend into each arm. The tube feet are joined with the radial canal through the lateral canal. Each tube foot has a bulging structure called the ampulla and a valve is present between the tube foot and the radial canal (Nichols 1967; Ruppert, Fox & Barnes 2004). The presence of the valves is for the buildup of the hydrostatic pressure that is required for the movement of the tube feet (Nichols 1967). In addition, the madreporite that is located on the aboral surface of a sea star is also joined with the water vascular system via the stone canal (Ruppert, Fox & Barnes 2004).
Figure 1). A simplified version of the water vascular system, image adapted from 'Invertebrate zoology : a functional evolutionary approach' by Ruppert, Fox & Barnes 2004
Hemal System
The hemal system of a sea star consists of three major interconnected radial vessel sets.
1) The hyponeural hemal ring (or the oral hemal ring): it is joined with the hyponeural radial hemal vessels which branches into the five arms of the sea star (Nichols 1967; Ruppert, Fox & Barnes 2004).
2) The gastric hemal ring (or the pyloric hemal ring): it surrounds the pyloric stomach (Nichols 1967; Ruppert, Fox & Barnes 2004).
3) The genital hemal ring (or the aboral hemal ring): it connects with the genital radial hemal vessels and finally leads to the gonads situated in each arm (Nichols 1967; Ruppert, Fox & Barnes 2004).
Figure 2). Diagram showing the internal anatomy of the hemal system, image adapted from 'Invertebrate zoology : a functional evolutionary approach' by Ruppert, Fox & Barnes 2004
Nervous System
The central nervous system of a sea star consists of a central nerve ring and five radial nerves that run through each arm (Nichols 1967; Ruppert, Fox & Barnes 2004). In addition, sea stars also have a sensory ectoneural system (in the epidermis) and a motor hyponeural system that is found in the coelomic lining (Ruppert, Fox & Barnes 2004).
Digestive System
The digestion system of a sea star begins with a mouth that situated in the center of the oral (ventral) surface. The mouth is connected to a short esophagus which then leads to a large cardiac stomach and a small pyloric stomach (Nichols 1967; Ruppert, Fox & Barnes 2004). The cardiac stomach is eversible and is used for extraoral feeding (Laxton 1974; Nichols 1967; Ruppert, Fox & Barnes 2004). The pyloric stomach extends into the arms of a sea star and the branched extensions are called the pyloric ceca (or hepatic ceca) (Nichols 1967; Ruppert, Fox & Barnes 2004). The pyloric stomach is then linked to a short intestine which eventually leads to the rectum and the anus that opens on the aboral side of the animal (Nichols 1967; Ruppert, Fox & Barnes 2004).
Figure 3). A simplified version of the digestive system, image adapted from 'Invertebrate zoology : a functional evolutionary approach' by Ruppert, Fox & Barnes 2004
Reproduction
Linckia laevigata is able to reproduce both asexually and sexually.
Asexual reproduction
Species of the genus Linckia is known to be capable of casting off part of their body when under threat/stress (i.e. autotomy) (Ruppert, Fox & Barnes 2004). The blue sea stars, which belong to the genus Linckia, have remarkable regenerative ability though have not yet shown to undergo autotomy (Williams 2000).
Sexual reproduction
Linckia laevigata is gonochoristic. In other words, they have separate male and female individuals (Yamaguchi 1977). Two gonads are situated on either side of the ambulacra within each arm and each gonad is connected to its own gonophore (Boolootian 1966; Lawrence 1987). Gametes (sperms or eggs) are discharged through the gonophores which open on the aboral surface (Boolootian 1966; Ruppert, Fox & Barnes 2004).
Biological body systems of a sea star and their physiological functions
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Functions
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Water Vascular System (WVS)
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Locomotion, feeding, gas exchange, respiration, internal transport (circulation), excretion
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Hemal System
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Internal transport, respiration, excretion
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Nervous System
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Sensory, muscle coordination, locomotion etc.
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Digestive System
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Feeding and digestion
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Reproductive System
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Reproduction
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