Phyla Mollusca and Echinodermata, Biology tutorial

General Features of Phyla Mollusca and Echinodermata:

The mollucs are unsegmented, triploblastic coelomates. They are generally bilaterally symmetrical. They are generally soft bodied, fleshy and separated in ventral muscular foot, head, and the dorsal visceral hump. Skin of hump secretes the calcareous shell. Central body cavity is haemocoel. There are no limbs. Mollucs include highly enhanced digestive tube. They also contain heart and blood circulatory system. Mollucs are usually sluggish except for oceanic squids. The body keeping operations are utilized to maintain large masses rather than high level of activity. Mollucs are thought to be deduced from basic annelid plan like arthropods but in different way.

Adaptations and Development:

1) The Shell:

The shell is laid down by outermost layer of epidermis of mantle and dorsal surface of body. They are produced in 3 layers. The outer thin horny coat, periostracum that is mostly made of protein and dissimilar chemically from chitin. It protects original calcareous shell from get dissolved by carbonic acid. Middle layer is prismatic layer composed of calcium carbonate in form of calcite or aragonite crystals. This layer is secreted beneath the thickened edge of the mantle. The third layer is the pearly or nacreous layer which is laid down by the whole of the outer surface of the mantle in form of thin sheets of calcium carbonate parallel to surface of shell. So shell keeps on increasing both in thickness and in extent. When mantle surface forms concentric layers of nacreous material around the foreign body like parasite or grain of sand, a pearl is created. The shell is the form of calcium carbonate absorbed by general body surface, mantle and gut. It passes by mantle tissue mostly as calcium phosphate. It is then deposited as the fibrous organic matrix outside mantle. Afterward it is converted in calcium carbonate. Molluscs also concentrate strontium - that could be radioactive - from nuclear - blast fallouts so radioactive strontium also gathers in molluscan tissues utilized as food by men and fish.

2) Locomotion and Loss of Segmentation:

Molluscs move by peristaltic waves of muscle movement passing by muscles at foot. In this arrangement both segmentation and lateral appendages are inappropriate. Additionally mollusks require space internally to contain digestive and circulatory system that have been foreshortened by shortened and broad body. Internal septa and body cavity divided by them have disappeared. The haemocoel like that found in arthropod has taken its place. Within haemocoel digestive tube has turned out to be long, coiled bulky and more localized. Heart is comparatively large. There are also within haemocoel excretory and reproductive organs.

3) Digestive System:

The alimentary canal is wiled at hinder end. It occupies most of haemocoel and is at points extended and is at points extended to form digestive glands and stomach. There is also a mouth prepared with radula that is thought to have initiated from muscular producible anterior end of annelid worms. Radula comprise of ribbon of chitinous teeth arranged in transverse and longitudinal rows. As animal crawls slowly along, radula is projected rhythmically it rotates also in process scraping off any soft tissue on the path. Finely ground particles of food are passed back through narrow gut to stomach. Here food particles are organized. Finer particles are sent into digestive glands while larger ones are passed on to intestines. Finer ones are absorbed into branched walls of intestine where digestion happens in tracellularly. Larger food particles unemployed in absorption are whirled up in mucous secretion and ciliary actions along intestines to anus. Absorptive part proper is digestive gland, with its huge branching ends into haemocoel.

4) Circulatory, Respiratory and Excretory Systems:

There is heart with openings that receives blood from haemocoel on either side through pair of lateral ostia. As heart contracts, valves close ostia, and blood is forced through arteries forward and backwards in body. Oxygen is attained and carbon dioxide removed as blood passes through gills that extend from both sides of body within mantle cavity of body. Gills are protected by mantle fold and suspended in flowing water. Gill is some feather-like extensions of gall body wall, richly supplied with 5 blood vessels. Large surface area given by gills provides gaseous exchange between blood and water. Blood have copper-base hemocyanin, as oxygen carrying pigment. Few aquatic snails have pigment hemoglobin though.

5) The Nervous System:

Molluscan nervous system comprises of the ring nerves surround mouth. From this ring long pair of nerves supplies muscles of foot, mantle, digestive tube, and other body structure. In squid, ring of nerves has become genuine brain; compact, complex and largest comparative to size of body among invertebrates. Nerve centers are concentrated in head region. Few are enlarged in keeping with high growth of motor and sensory activity of whole body. Pair of optic lobes has become so elongated in comparison to enormous squid eyes. There is also large central lobe for coordinating activities of whole brain and for storage of information in form of memory. Squid has the strikingly developed eyes with transparent lens, corner, a pigmented ivis diaphragm, darkened eye chamber, several-layered light sensitive retina and supportive tissues.

Classification of mollucs:

There are three classes of mollucs.

1) The Gastropod:

It has real head equipped with brain or cerebral ganglia, pair of eyes, organs for balance (statocysts) and tentacles on top and in front of radula bearing mouth. Respiratory system is elongated and simplified with only 1 or 2 gills in a large surrounded mantle cavity. Water is drawn into the mantle through the funnel-like fold of mantle itself. In gastropods large digestive gland and adjacent tissue grow in the spiral manner causing rear end openings to be carry forward. They can be carnivores or herbivores.

2) Bivalves:

They are second group of mollucs. Shells are separated into left and right halves joined by ligamentous hinge on dorsal side of animal. This 2 part shell only seems to be two. The hinge causing two shell appearances is the uncalcified part of the whole. So shell is in actuality only one. Two haves have elongated to enclose whole animal. They are pulled together by strong adductor muscles inserted in inner surface of shell. When wholly covered by shell bivalves are protected against most preys except starfish, carnivorous whells and snails that drill through the shells with the radula. Bivalves more about very little. They get oxygen and food particles from water. They body is compressed from side to side to permit left and right shells to meet each other on ventral side when closed so also underlying mantle that secretes shell by which it is protected.

3) Cephalopods:

They are fast-swimming mollucs. Squid captures its prey, fish in full motion and also avoids being eaten at same time by moving with highest speed, by water. To do this, squid require power and slightest resistance to water. It also requires coordination and sensitivity to manage direction when speeding. The squid has the streatined body and the well grown head with sense organs, large brain and 10 prehensible tentacles. It has powerful hydraulic jet propulsion locomotor system and fins. It has internal distributive system to fulfill these new demands. Initially, squids had to decalcify and turn light for their kind of living.

General Features of Echinoderms:

Echinoderms are usually triploblastic, coelomates. They all are marine kinds. The adults are pentamerously radially symmetrical. Their organ of locomotion is tube feet. They also contain calcareous endoskeleton. They have no head. Mouth is usually on lower (oral) surface of body while anus is on upper (aboral) surface of the body. There are 2 classes of echinoderms, stelleroidisa (starfish) and echinodermata (sea urchins). Starfishes are star-shaped and flat. Arms of star are not sharply separated from disc. They contain few calcareous plates in the body wall. Their spines are variable. Conversely Echinoderm is of globular in shape. They don't have arms. Numerous calcareous plates in body wall are joined to each other to form the rigid movable structure.

Symmetries of Echinoderms:

Bilateral symmetry is the features of mobile animals. Associated to this is directed locomotion of 2 sides of body. This is also associated with mouth for food intake and the head with sensory organs situated at the advancing end. Conversely sessile animals or those which drift without direction are more efficient on the radial symmetry plan. Echinoderm structure as demonstrated has the 3 part plan that represents the biological device which functions as follows: there is stalk (root-like basal process) that anchors organism to substratum and lifts rest of body above turbid bottom water. Second part is 5 pairs of puriate tentacles radiate outward and upward around mouth and serve to collect food particles and bypass them to mouth.

Conversion to Mobility:

All free living echinoderms show radial symmetry and have unique water vascular system. Starfish has its mouth and ambulacral side of its arms face down. Increased mobility is the reply to accessibility of food. The five arms of starfish radiate. Body and arms are quite rigid due to calcareous plates embedded in tissue of calcareous wall. Mouth is in centre of undersurface. Anbulacral groves expand along underside of every arm from mouth to hip. Podia in starfish are known as tube feet. Tube feet extend in series along ambulacral grove. Conversion mobility has to do with change in use of podia and water-vascular system, operating in association with ambulacral groves to serve as locomotory mechanism instead of the feeding mechanism. Water-vascular system turns into hydraulic power system. Water is drawn into system by ciliary action through minute surface. This passes down the tube (store canal). This tube is hardened by calcareous rings to ring canal surrounding mouth. From ring canal, 5 radial canals lead off, one into every arm and every radial canal provides off the series of lateral branches to tube feet of each side.

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