Essay on Characteristics of Molluscs

Molluscs

Molluscs portray a remarkable amount of diversity. Most of them have specific features in common. The word mollusc originates from Latin which means soft. Molluscs are mostly found in water bodies. Living in the water bodies exposes them to many predators because their bodies are easily eaten. Due to this reasons, it has several adaptive features to enable them to survive attacks from the predators. It has developed hard shells for hiding purposes from the predators. For instance, cone snail has cone-shaped shells and sport large which supports in drilling into the ocean sand. Furthermore, the model of the shell set hurdles for the predators to grasp their inclined lengths.

Abalones

Abalones too find protection from the shells. When the bigfoot muscle of Haliotis compresses on a rock, it is difficult to separate the abalone without causing harm to it. It means that the abalone is likely to be entirely guarded against harm by the octopuses and sea otters. However, it cannot escape from sunflower stars because they digest their prey if the abalone does not crawl away. It is always challenging to have a view on abalone against the background of algae on the rocks where it is their habitat. It is because the shell is coated with filamentous algae and also pink encrusting coralline algae. The color in an abalone shell originates from bilin-type pigments obtained from seaweed floods which are included in the proteinaceous external layer of the shell.

Feeding Mechanisms

Molluscs have different feeding mechanisms. In the case of bivalve molluscs, they sieve and feed the light elements from water. It is different in a limpet which is made up of a single-shell. It has a ribbon-shaped radula wrapped with rasping teeth. It facilitates the animal to scrape algae from the rock. Whelks have radula on a stalk which can enlarge past the shell and also used in boring into the shells of other molluscs. Furthermore, the shells contain a stalked radula which is changed into a kind of harpoon which they use to acquire their pray before poison is injected into them.

Maintaining Vertical Position

Sea animals have a challenge of maintaining a vertical position. Molluscs such as Sepia, Nautilus, Loligo or squid, and Octopus get support from being neutrally buoyant. The energy is not frequently expended to work against body weight. Compared to other animals, squid also depends on the speed of movement to maintain floating or to obtain a neutral buoyancy. An animal is required to have a lot of muscle protein to make the body denser than water to sustain perfect buoyancy. However, some animals prefer having lower protein-content muscles which are watery and lighter.

Color Adaptation in Squid

An animal such as squid uses color for survival. It applied for camouflaging or to caution the predators in their undersea realm. It can change color in a blink of an eye. It has arms which are attached to the head. Furthermore, they can change their color to resemble their environment making them invisible or make them stand out from the surrounding. It contains numerous cells which change colors known as chromatophores. They are located on the surface of the skin that facilitates changes in color. Every chromatophore has a flexible sac full of pigment situated at the center. The different collection of nerves and muscles directs the expansion and contraction of the sacs. The animal changes color to hide from the predators.

Echinoderms

Echinoderms consist of distinctive endoskeleton which is a strange organ system called the water vascular system with several other less unique features. The endoskeleton entails a meshwork of plates and spines which are joined by the mesodermal tissue. The meshwork of plates is called ossicles. The ossicles consist of a microscopic system of calcium carbonate crystals which creates a structure called the stereo. The echinoderm endoskeleton has many functional characteristics which differ between species. For instance, pedicellariae which are found on sea stars and sea urchins are modified as pincer-like structures to obstruct predators.

Water Vascular System

The water vascular system of echinoderms entails a structure of fluid-filled canals that enlarges alongside all body parts. It also has several external protrusions called tube feet. The water vascular system has many functions. For instance, it uses the water pressure to control movement and supports feeding. The vascular systems do Excretory, respiratory and many circulatory functions in the animal. The water vascular system consists of the stone canal, radial canals, tube feet, madreporite, and circular ring canal. Each organ has a unique function in the body. The stone canal connects the circular ring canal with madreporite. It allows water to flow between the two parts. The radial canal has several extensions which are connected to different body parts through a groove called the ambulacral groove. The tube feet stretch between the endoskeletal plates to the external of the organism. The madreporite is a sieve-like calcified plate whose function is it connects the system to the aquatic environment.

Sea Star

The sea star which falls under echinoderms has a radial symmetry with many arms originating from the center of the body. The mouth and anus are located adjacent to each other on the lower side. The anus is located in the middle of the disc in common with madreporite. The superior part of the animal is regularly multicolored. Minute pincer-like structures known as pedicellaria are mandated to make sure the exterior of the arms is free from algae.

Digestive Tract in Echinoderms

Most of the echinoderms have a complete and straightforward digestive tract. It is seen in a starfish where the mouth is found below the hub of the disc. The alimentary canal entails a large stomach that takes much space of the central disc. It also has elongations which project alongside every arm. Starfish are carnivores and scavengers. Indeed they feed on invertebrates especially the snails, bivalves, and marine worms. Mostly, they are attracted to dead animals bodies found on the seafloor. However, some have unique eating habits and prefer feeding on cucumbers while others have adapted to eat a variety of prey. The sea urchins have an enhanced feeding structure known as Aristotle's lantern. Structurally, it has the shape of a pyramid where the teeth are located at the point where it faces down from the middle of the sea urchins body. The sea urchins are habitually grazers. For instance, they feed on algae such as kelp and grass. However, some will eat corals and others on minute food particles found in sand.

Chordates

Chordates are a diverse group of animals which consists of the phylum Chordata. It is estimated to be 44,000 species of chordates. The sizes vary depending on the species. Pre-vertebrate animals such as tunicates, amphioxus and tunicates are the earliest and simplest chordates. The main class of chordates is the subphylum Vertebrata. It exhibits a bilateral symmetry with a body cavity that is enveloped within a membrane called peritoneum. It originates from the inner tissue layer known as the mesoderm. A structure known as notochord defines chordate. It is a rod-like, elastic structure located along the exterior, mid-line of chordates. All chordates have a notochord at least at one stage of their life. The notochord hardens the body against the pull of muscles in the earliest chordates. However, this purpose is inferior in the more advanced vertebrate chordates. Indeed, it is because, in the developed vertebrate, their bodies are sustained by the body cartilaginous and bony components of the skeleton. In vertebrates, the notochord exists during the embryonic, developmental stages.

Unique Features of Chordates

Chordates have several unique features implying that there has been a massive transformation from the simple beginnings. During the early stages of chordate, growth shows features which are shared with some invertebrate phyla. It occurs specifically the mouth which develops separately from the anus. One of the unique descriptions of the chordates is the presence of a notochord. It is an elastic, rod-shaped structure found when most of the chordates are in the embryonic stage and the adult stage of some of the species. It is located in the digestive tube and the nerve cord thus facilitating skeletal pillar in the body segment. In other chordates, the notochord functions the same as the primary axial which supports the body during the animal's life. In vertebrates, the notochord is present during the development of the embryo. During this time, it stimulates the growth of the neural tube which functions as the foundation of the growing embryonic body. However, it is substituted by the vertebral column in most of the adult vertebrates.

Dorsal Hollow Nerve Cord and Pharyngeal Slits

Another unique structure in chordates is the dorsal hollow nerve cord. It originates from the ectoderm that spins into a hollow tube during growth. Furthermore, it is located on the upper side of the animal to the notochord. The nerve cord established in the majority of the chordate embryos grows into the brain and spinal cord which consists of the central nervous system. Despite the nerve cord, chordates also own pharyngeal slits. They are gaps in the pharynx which stretches to the outside surrounding. In aquatic animals, pharyngeal slits permit for the removal of water which flows through the mouth when feeding. Some of the invertebrate chordates have different functions of the pharyngeal. They use the slits to sieve food from the water which enters the mouth. In most terrestrial animals, mammals and birds being included, their pharyngeal slits are present only during the embryonic development. In this group of animals, the pharyngeal slits grow to become jaws and inner ear bones.

Post-Anal Tail

The post-anal tail also forms part and characteristics which define chordates. It is an external extension of the body past the anus. The tail consists of skeletal elements as well as muscles. The primary function is to facilitate the source of locomotion in the aquatic species. In some of the terrestrial species, the tail is used for balancing purposes. It is also used for courting and signaling when they sense danger. Also, some chordates such as fish have highly effective swimming capabilities. The multiple fins found on their body enables them to maneuver, thrust, maintain stability and allow them to take breaks. The fish can turn up and down by just flexing the pectoral and caudal fins. Furthermore, pectoral fins purpose is for breaks. The location of the paired fins that is the pelvic fins are persistently adjusted to maintain the fish from pitching or rolling.

Hemichordates

Hemichordates form a small group of the phylum. It is any of a group of wormlike marine invertebrates directly associated with both the chordates and the echinoderms. Usually, they are thought to represent a phylum. Molecular studies have shown that the hemichordates are more likely linked to echinoderms more than chordates. It entails three classes, which include planctosphaeroidea, enteropneusts, and pterobranchia. Planctosphaeroidea is recognized from a few floating larvae. Enteropneusts are widespread in the intertidal zones from the White Sea to the New Zealand and the Cape of Good Hope. Pterobrancia which entails about twenty species is colonial, minute and tube-building forms. An adult hemichordate body and cavities are classified into three parts. It includes the collar, truck, and proboscis. Hemichordates do not have a central nerves system. However, the nerve system is focused on the collar.

Circulatory System and Digestive Tract in Hemichordates

Hemichordate has a circulatory system. It consists of blood vessels, sinuses and contractile heart like vesicle. The blood system is found between the two lamellae of the underside membrane of the body epithelium. It has two main blood vessels which are dorsal and ventral longitudinal vessels. The heart vessels have no blood. However, it is muscular, and the contractions pump blood in the central sinus into the glomerulus. The central sinus is non-contractile, and the endothelial lining is regularly incomplete. The blood is ordinarily colorless with a few cells present.

The digestive tract entails a straight epithelial tube with little or no intrinsic musculature. It lacks longitudinal muscle fibers in the gut wall. The big mouth is located in the ventral collar that is between the collarette and the probosci's stalk. It opens into a buccal tube occupying the center of the collar interior. The buccal system leads to the esophagus which later directs to the anterior intestine in the hepatic region. The anterior guts lead to the hind intestines which are part of the rectum. The entire luminal epithelium of the gut is ciliated thus enabling movement of food.

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