Basic Concept of Invertebrate Zoology Knowledge

Thursday, 18 April 2013

Types of Coelom

A true coelom is lined on all sides by mesoderm which gives rise to muscles that surround the gut aas well as underlying the body wall. This allows more efficient digestion because food can be pushed through the digestive tract by muscles.

True Coelom :-

Schizocoeloms are found in animals known as Protostomates.

Enterocoeloms are found in animals known as Deuterostomates.

Protostomia :-

Cleavage is spiral. In spiral cleavage the first two divisions of the egg are equal, but the rest are unequal. These results into some small and some large cells, as it can be seen. The blastopore forms the mouth. The body cavity is formed by Schizocoely- Schicoely means that the body cavity is formed from blocks of mesoderm around the gut that enlarge and hollow out. The end result is a pair of coelomic cavities. Cleavage is determinate. This means that the fate of each individual cell is already established when it is formed. Each cell can only develop into a specific cell type.

Deuterostoma :-

Cleavage is radial- Each cell division is equal and result in cells of all the same size. The blastopore does not form the mouth, it usually forms the anus. The body cavity is formed by enterocoely. Enterocoely is the formation of the body cavity by out pocketing of the primitive gut that break off and form the coelom. Cleavage is indeterminate. This means that the fate of each individual cell is not determined when it is formed and each cell can form one of many cell types. There are cavities , coeloms. that form and lie both outside (extraembryonic) and inside (intra-embryonic) the embryo during development. The intra-embryonic coelom is the cavity that lies within the developing embryo that will form the 3 major body cavities : Pericardial, Pleural. Peritoneal. The coelom forms very early in embryogenesis and is much later partitioned inferiorly by the diaphragm and pleura-peritoneal membrane ; and superiorly initially by the pleuro-pericardial fold between the heart and lungs. The intraembryonic coelom communicated through coelomic portals (at the level of midgut herniation) with the extraembryonic coelom. All cavities are fluid filled and developing organs push against a wall of the cavity, generating a double coat (serosal/adventital) surrounding an organ (for example the lungs). The serous membrane is the epithelium (squamous) and its associated underlying loose connective tissue.

Protostomes Vs Deuterostomes :-

In echinoderms and chordates, for example, the anus forms first, and then the mouth. In animals belonging to several other important phyla, on the other hand, such as arthropodes, annelids and molluscs, the mouth forms first, followed by the anus. For this reason arthropods, annelids and molluscs are sometimes called Protostomes (Greek : proto = the first ; stoma = mouth ). It is also possible to take an most echinoderm or chrodate embryos at the 2-cell or 4-cell stage of development, separate the cells, and still have each cell continue on to develop into a complete, variable organism. This is not possible with protostome embryos. If they are separated at this stage, the cells will not develop into complete, viable organisms. For this reason, protostomes are said to display determinate cleavage, and deuterostomes are said to have indeterminate cleavage. You will be introduced in lecture and in subsequent laboratory exercises to other important developmental characteristics that distinguish protostomes from deuterostomes.

Advantage of a coelom :-

(i) The hydrostatic skeleton a fluid body cavity provides a more rigid structure than mesoderm for muscles to pull against and thus be a more efficient skeletal support system.

(ii) Circulatory system : absorbed nutrients can be circulated around the body and metabolic wastes can be carried to the body surface more efficiently by fluid than by solid mesoderm.

Result- Faster locomotion and larger body size possible.

Advantage of Pseudocoelom :-

In evolution, the pseudocoelom is a blatocoel (the space in the blastula) that is retained to adulthood. In addition to having a body cavity, organisms with pseudocoeloms also have a complete digestive tract separate opening for food to enter and undigested material to leave. This makes digestion and feeding more efficient because the animal can eat before it has finished digesting its previous meal. Organisms that have pseudocoeloms are not a monophyletic group (in other words, the pseudocoelom is a homoplasious character that occured more than once).

Friday, 12 April 2013

Types of Coelom

Pseudocoelom :-

In some protostomes, the embryonic blastocoele persists as a body cavity. These protostomes have a fluid filled main body cavity unlined or partially lined with tissue derived from mesoderm. This fluid-filled space surrounding the internal organs serves several functions like distribution of nutrients and removal of waste or supporting the body as a hydrostatic skeleton. A pseudocoelomate is any invertebrate animal with a three-layered body and a pseudocoel. Nematoda (roundworms), Rotifera (rotifers), Kinorhyncha, Nematomorpha, Nematomorphs or horsehair worms, Gastrotricha, Loricifera, Priapulida, Acanthocephala, spiny-headed worms, Aschelminth animals, Entoprocta are the examples of Pseudocoelomates. The coelom was apparently lost of reduced as a result of mutations in certain types of genes that affected early development. Thus, pseudocoelomates evolved from coelomates. Pseudocoelomate is no longer considered a valid taxonomic group, since it is not monophyletic. however, it is still usd as a descriptive term.

Important characteristics as the psudocoelomates are adated to survive that they lack a vascular blood system, as a diffusion and osmosis circulate nutrients and waste products throughout the body. They lack a skeleton hydrostatic pressure gives the body a supportive framework that acts as a skeleton. Furthermore they have no segmentation. Their body wall comprises of epidermis and muscle often syncytial usually covered by a secreted cuticle. They are mostly microscopic and chosen a parasite mode life (although some are free living ).

Acoelomate :-

Lacking a fluid filled body cavity presents some serious disadvantage. Fluids do not compress, while the tissue surrounding the organs of these animals will compress. Therefore, acoelomate organs are not protected from crushing forces applied to the animal's outer surface.

Organisms showing acoelomates formation include the platyhelminthes (flatworms, tapeworms, etc), the Cniderians (jellyfish and allies), and the Ctenophores (comb jellies). The coelom can be used for diffusion of gases and metabolites etc. These creatures do not have this need, as the surface area to volume ratio is large enough to allow absorption of nutrients and gas exchange by diffusion alone, due to dorsoventral flattening.

The coelom is defined as a body cavity or space than runs the length of a vertebrate. It separates the body into an inner tube and an outer tube. the coelom forms when the lateral plate mesoderm splits.

Evolution of Coelomate Animals :-

In the Cniderian, the space between the ectoderm and endoderm tissue layers is filled with an acellular mesoglea. In the platyhelminth (flatworms) these mesoderm fills the space between mesoderm and the endoderm :

The advantage of the flatworm body plan is the superior pull muscles can make using solid mesoderm as a lever, versus gel-like mesoglea. But, body organs can not move freely but are embedded in solid mesoderm tissue. It is not more difficult for materials to move from the gut to the body wall. Probably to solve the problems the is to have some sort of body cavity, called a coelom, in which the body organs lie bathed in body fluid. Movement of the body wall would not squeeze the organs, organs could glow without being pressed by the body wall and diffusion of nutrients and wastes would be easier.

Tuesday, 9 April 2013

Introduction & Significance of Coelom

Introduction :-

A coelom is a fluid-filled body cavity found in most animals which is located in the mesoderm, derived from the splanchnic mesoderm, the middle germ layer only found in tiploblastic (three-layered) organisms. Simpler animals like Cniderians (ex- Jelly fish, Coral etc ) and sponges are diploblastic and monoblastic respectively, lacking a coelom. Though the coelom developed in tiploblastic animals, some of these animals have lost their coeloms.

Animals with a coelom, including the majority of animal phyla, are called coelomates. Animals without a coelom, such as flatworms, are called acoelomates. In between there are some animals called pseudocoelomates, which possess a "false coelom," which is an unlined or partially lined body cavity between the gut and body wall.

The earliest known animal with a coelom is vernanimalcula guizhouena, which lived 600 million years ago, during the Ediacaran period. The biggest disadvantage of lacking a coelom is that the internal organs are much more susceptible to compression and damage. The protection that coelomate animals get from their body cavity stems from the fact that fluids are incompressible, while organs are not. Since the organs are surrounded by an incompressible fluid, they are well-protected in coelomates.

The coelomates nonchordate phyla comprise Entoprocta, Ectoprocta, Phoronida, Brachiopoda, Mollusca, Priapulida, Sipuncula, Echiura, Annelida, Tardigrada, Pentastoma, Onychophora, Arthropoda, Pogonophora, Echinodermata, Chaetognatha, Hemichordata.

However, In contrast a lots of and numerous animals are pseudocoelomate, including nematodes, rotifers, kinorhynchans (mud dragon), nematomorphs (horsehair worms), gastrotrichs, loriciferans, priapulid worms, spiny-headed worms, and entoprocts. Many of these organisms are less than a millimeterin size, but some, like priapulid worms, grow as large as 6 inch (15 cm).

Significance :-

(i) A coelom is defined as a cavity that separates the gut from the body wall. The coelom allows the internal organs to shift around and develop independently of the body wall, creating more physiological and evolutionary flexibility. Though the coelom usually refers to the largest body cavity, coelomates animals may feature several strategically placed body cavities.

(ii) Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions and protects them from shocks. Arthropods, mollusks even certain annelids have a reduced coelom. This condition in various ways may be compensated by other alternatives. As in former two groups principal body cavity is the hemocoel of an open circulatory system. In leeches the botrydoial tissue do the work as coelom is supposed to do.

(iii) In the past, zoologists grouped animals based on characters related to the coelom. The presence or absence of a coelom and the way in which it was formed was believed to be important in understanding the phylogenetic relationships of animal phyla. However, recent molecular phylogenies have suggested this characteristic is not as informative as previously believed : the coelom may have arisen twice, once in protostomes and once among the deuterostomes.

Formation :-

Coelom formation begins in the gastrula stage. The developing degestive tube of an embryo forms as a blind pouch called the archenteron.

In Protostomes the coelom forms by a process known as schizocoely. The archenteron initially forms, and the mesoderm splits into two layers : the first attaches to the body wall or ectoderm, forming the parietal layer and the second surrounds the endoderm forming the visceral layer or alimentary canal. The space between the parietal layer and the visceral layer is known as the coelom or body cavity. In Deuterostomes, the coelom forms by enterocoely mesoderm buds from the walls of the archenteron and hollows to become the coelomic cavities.

Origins :-

The Acoelomate Theories :

Coelom evolved from an acoelomate ancestor.

The Enterocoel Theory :

Coelom evolved from gastric pouches of Cniderian ancestors.

Supported by research on flatworms and small worms recently discovered in marine fauna ("Coelom").

Types of Symmetry

Spherical Symmetry :-

Spherical symmetry is also called Homaxial apolar symmetry. Spherically symmetrical forms are best suited for floating and rolling. The rare type of symmetry is spherical symmetry. Spherical symmetry is found in Heliozoans and radiolarians. Body of the spherically symmetrical animal can be cut into two identical halves in any one of the infinite number of planes that pass through the central point. Body parts are arranged concentrically around or radiating from a central point in spherical symmetry.

Radial Symmetry :-

Radial symmetry is also called monaxial heteropolar symmetry. In radial symmetry the sensory and feeding structures are uniformly distributed around the body hence they can interact with their environment equally in all the directions. In radially symmetrical animals body is cylindrical (ex- Hydra), or vase- like ( sponges) or umbrella shaped (ex- Jelly fish). In Radially symmetrical animals the principal axis is oral-aboral axis it is heteropolar. Anterior, posterior ends and dorsal, ventral, lateral surfaces are not differentiated in radially symmetrical forms as well as spherically symmetrical forms. Radial symmetry is found in some sponges and most of the Cniderians. Echinoderms are primarily bilateral animals (their larvae are bilateral). Most of the echinoderms have secondarily become radial, exhibiting pentamerous radial symmetry in the adult stage.

Biradial Symmetry :-

The Biradial type of symmetry of sea anemone seems to have been derived from the radial type, which is primarily by the elongation of the mouth and associated parts. Biradial symmetry has two planes of symmetry. Ctenophores and most anthozoans exhibit biradial symmetry. In ctenophores most of the body parts radially arranged but the tentacles are paired. A sea anemone, with two siphonoglyphs does exhibit biradial symmetry. While a sea anemone with one siphonoglyph exhibit radial symmetry.

Bilateral Symmetry :-

In Bilateral symmetry the principal axis is the anterior- posterior axis, it is heteropolar, with differentiate anterior and posterior ends. In Bilateral symmetry sagittal axis is heteropolar and the transverse axis is apolar. In bilatearl symmetry there is only one planes of symmetry, it is median sagittal plane. The appearance of bilateral symmetry was a major advancement in animal evolution. Cephalization is associated with bilateral symmetry. As a result of cephalization, bilaterally symmetrical animals can sense the new environment into which they enter and respond to it more efficiently than other animals. All the tiploblastic animals of animal kingdom exhibit bilateral symmetry.

Evolutionary advantages of Bilateral Symmetry :-

Here parts of bodies can develop differently; different organs can be located in different body parts.
Here the movement made more efficient (direction of movement better defined, resistance to water or sediments reduced).
It brings higher mobility as higher flexibility becomes a key factor for more efficient in seeking food, finding mate, avoiding and escaping predators.

Introduction of Symmetry

Animal Organization :-

The evolution of tissues is the first key transition in the animal body plan. Eumetazoans exhibit higher levels of organization. The embryonic tissue layers of eumetazoans are called germ layers.

The lowest level of organization is cellular grade of organization, which is exhibited by sponges.

Division of labor seen among the cells in sponges and such cells have little tendency to become into tissues. Nerve cells and sensory cell are absent; hence cells are functionally isolated in sponges. In cellular grade of organization cells demonstrate division of labor but are not strongly associated to perform a specific collective function.

Diploblastic animals such as cniderians exhibit tissue grade of organization or diploblastic organization. In tissue grade of organization germs layers such as ectoderm and endoderm are developed. Cells are aggregated inti tissues. Nerve cells and sensory cells are present in the epidermis and the gastrodermis. Ectoderm gives rise to the epidermis; endoderm gives rise to the gastrodermis. The cells of a tissue together perform their common functions as a highly coordinates unit. Tiploblastic animals exhibit organ system grade of organization or tiploblastic organization. In organ-system grade of organization all the three germ layers are present such as ectoderm, endoderm and mesoderm. The evolution of mesoderm resulted in structural complexity. The tissues are assembled into larger functional units called organs. Organs working together to perform one or more specific functions constitute an organ system. Eleven kinds of organ systems are observed in metazoans such as integumentary, skeletal, muscular, digestive, respiratory, circulatory, excretory, nervous, endocrine, immune and reproductive. Tiploblastic animals have highly specialized sensory cells and nerve cells which bring about a higher level of coordination and integration.

Symmetry :-

The symmetry of an animal generally fits its life-style. Radial symmetry is an advantage to sessile or planktomic or slow moving organisms. Evolution of bilateral symmetry is the second key transition in the animal body plan. In symmetrical animals are body lacks definite form or geometrical arrangement of parts and can not divided into mirror image halves by any plane. Gastropods are primarily bilaterally symmetrical and secondary asymmetrical. Most of the sponges and some protozoan like Amoeba exhibit asymmetry. In a symmetrical animal similar body parts occur on the opposite sides of a plane passing through the principle axis of the animal. A symmetrical animal can be cut into two mirror image halves or antimeres by one or more planes of symmetry passing along the principle axis. The term axis refers to an imaginary straight line joining two opposite points at the ends, or on the surfaces of the body. The term principle axis means an imaginary straight line joining the midpoint at one end or surface and the midpoint at the opposite end or surface. The term plane means a flat area that runs through any axis. Bilaterally symmetrical animals possess anterior (cephalic) end posterior (caudal) end dorsal surface, ventral surface and lateral surfaces. Frontral plane is the plane that runs through the anterior- posterior and sagittal axis. Transverse plane(cross section) passes through sagittal and transverse axis. Frontral plane divides a bilaterally symmetrical body into dorsal and ventral portions. Sagittal plane divides bilaterally symmetrical animal into right and left halves. Transverse plane divides the animal into anterior and posterior portions. Each of these three planes is at right angles to the other two planes.