Triploblastic Animals – The Acoelomates
Phylum: Platyhelminthes – The Flatworms
General Characteristics
The name platyhelminthes means “ flatworms ”. The body of these animals is soft and dorsoventrally compressed. The platyhelminthes are triploblastic acoelomates. There is development f a third layer, the mesoderm, which separates the ectoderm and endoderm. The platyhelminthes exhibit bilateral symmetry, and body is unsegmented.
With few exceptions the platyhelminthes are parasites, mostly endoparasites, i.e, live inside their hosts. The most common examples are Taenia solium (Tapeworm), Fasciola hepatica (Liver fluke) and Schistosoma (blood fluke). The parasites are more common in tropics. Some of these cause diseases in humans.
A few species are free living and found in freshwater, for example Dugesia (planaria). Their size ranges from few millimeters (10 mm in case of planaria) to several meters (tapeworm). Much of the body space is taken up by a branching sac type digestive system. The digestive system is poorly developed in some species or may be absent as in the tapeworms.
Excretory System
The excretory system consists of branching tubes ending in bulb-like cells, the flame cells. A well developed nervous system is present in platyhelminthes. It is in the form of either a simple network of nerves or ganglia. The sense organs are present at the anterior a simple network of nerves or ganglia. The sense organs are present at the anterior end. Respiratory and circulatory systems are absent.
Feeding
The parasitic species absorb nutrients from the hosts. The free-living species (planaria) feed on small animals and bodies of dead and decaying animals.The free-living forms are motile. They move by cilia present on their undersides (planaria). In parasitic forms the movement is restricted.
Reproduction
The platyhelminthes reproduce both by sexual and asexual means of reproduction. Asexual reproduction is by fission in which the animal constricts in the middle into two pieces, each of which regenerates the missing part. The sexually reproducing species are hermaphrodite, i.e., both male and female reproductive organs are present in the same individual. Larval form is sometimes present.
The common examples of flatworms are:
- Dugesia (Planaria): A free-living flatworms with a ciliated outer surface.
- Fasciola (Liver fluke): It is an endoparasite in sheep and occasionally in human beings. It has suckers used for attachment to host tissue. It completes its life cycle in two hosts, a snail, sheep or man. It lives in the bile duct of its hosts.
- Taenia (Tapeworm): An endoparasite of humans, cattle and pig, that completes its life cycle in two hosts. The intermediate host is pig or cattle. The body is ribbon-like and divided into segments called proglottids which contain mainly sex organs. The segments continue to break off and are passed out from the intestine along with faeces.
Adaptations for parasitic mode of life
The parasitic platyhelminthes have completely adapted themselves to parasitic mode of life by the development of the following characteristics:
- The epidermis is absent and there is the formation of resistant cuticle for protection.
- They have developed adhesive organs, such as suckers and hooks, for attachment to the host.
- There is degeneration of muscular system and nervous system.
- The digestive system has become simplified due to increased dependence on host.
- The reproductive systems are complicated and the ova are produced in huge numbers to ensure continuity of the species.
- The complexity of life cycle and presence of more than one host during the life cycle is also an important parasitic adaptation.
Infestation
In Taenia (tapeworm), the development of the zygote begins while it is still inside the uterus of female. The last segments or proglottids and their uteri contain completely developed embryo. The fully mature proglottids break of from the body and pass out of the body of man along with faeces undigested waste). The embryo inside the egg is round in shape and has six chitinous hooks. It shows limited movement of contraction.
In order to develop further it must reach a second host which may be a cow. The parasite remains embedded in the voluntary muscles of cow. If an improperly cooked beef is eaten by a person, the parasite which has not been killed begins to develop further in the intestine of man.
Disinfestation
Once the parasite has entered the intestine of man it is difficult to remove it completely. In this respect care should be taken to cook beef properly before eating it. So that there is no chance of the parasite entering the digestive system but if it has entered then certain medicines are taken to remove it. Its complete removal is necessary because if only head remains inside the intestine it can grow into new tape-worm once again. Besides treatment with drugs, physicians also give anema to the patient, to fully remove the parasite.
Triploblastic Animals – Pseudocoelomates – Aschelminthes (phylum Nematoda) – The Round worms
General characteristics: The name nematode means “pointed ends”. The animals included in this group have elongated worm like body with pointed ends. The nematodes are triploblastic and pseudocoelomates. One end of the body is anterior, however the head is not clearly marked and there are no special sense organs at this end. The nematodes exhibit bilateral symmetry and the body is unsegmented. The body cavity is pseudocoelom.
It is derived from the hollow space, the blastocoels, situated in the blastula, an early stage in embryological development, and not from the mesoderm. It consists of a number of vacuolated cells filled with a protein-rich fluid which develops high hydrostatic pressure.
The nematodes range from small microscopic forms, to some from reaching a length of up to one meter. The digestive system is in the form of alimentary canal with two openings. The opening at the anterior end is mouth and at the posterior end is the anus. In parasitic nematodes the digestive system is simple. A fluid filled space is present between the body wall and alimentary canal. It provides “tube within tube” type structure in nematodes.
The excretory system consists of two longitudinally running excretory through an excretory pore on the ventral surface. There is a nerve ring around the pharynx, which give rise to dorsal, ventral and lateral nerve cords running throughout the length of the worms. The sense organs are in the form of sensory papillae present on the lips at the anterior end. The circulatory and respiratory systems are absent. The gaseous
Exchange takes Pl through general body surface; locomotion is by undulating waves of contraction and relaxation of muscles. These muscles are arranged in four bands, two dorso-lateral and two ventro-lateral. The circular muscles are absent; therefore the bending is dorso-ventral only.
The sexes are separate. The female gonads are ovaries and these male gonads are testes which produce sperms. A larval stage is present in the life cycle.
Importance –Parasitic Diseases
Aschelminthes is important from the point of view of its parasites of which it has a great variety causing some very serious diseases in man and plants.
Ascaris dumbricoides is an intestinal parasite of man.
The genus Rhabditis contains numerous species normally found in soil, organic matter or water and feces of man or animals. Enterobius vermicularis commonly known as pin worm is cosmopolitan but more common in Europe and America. Pinworms are parasites in the human caecum, colon and appendix. Their movement causes intense itching of anus, inflammation of mucous membrane of colon and appendix resulting in insomnia and loss of appetite.
Ancylostoma duodenale is commonly known as hook worm. It is a parasite of human small intestine in Asia, North Africa and Europe. It is very dangerous because it holds the villi of intestine and sucks blood and body fluid. During feeding they produce an anticoagulant to prevent clotting of blood and after feeding leave the wound bleeding in children it can cause severe anemia and retard physical and mental growth.
- Round worms are everywhere outdoors, where they play an important role in breaking down organic matter. A single rotting apple may contain 90,000 worms. Billions thrive in each acre of topsoil.
Triploblastic Animals – Coelomates
Phylum Annelid the Segmented Worms
General Characteristics
Most of the worms with which we are familiar are included in this phylum. They are segmented and commonly called annelids. (From the Latin word for “little ring”)
The body is metamerically segmented. The body becomes divided transversely into a number of similar parts or segments. The subdivisions may be indicated externally by constrictions of the body surface. Internally, the segments are separated from each other by septa extending across the coelom. However, the various systems of body such as gut, blood vessels, and nerve cord are continuous throughout the length of body penetrating each individual segment.
The animals are triploblastic and coelomate, showing bilateral symmetry.
The annelids include worms, which may be marine (nereis), freshwater (stylaria) or found in damp soil (earthworms). Some are parasites, for example, Hirudo – leach).
The annelids show specialization of body structures. The organ systems are well developed.
Digestive system is in the form of alimentary canal which is divided into distinct parts, each performing a specific function. It has two openings, the mouth at the anterior end, and the anus at the posterior end. The mouth is overhung by a lobed structure, the Prostomium. In parasitic species, the digestive system is poorly developed. Annelids have true coelom i.e. the mesoderm splits into parietal layer which lines the body wall, and the visceral layer wwhich covers the alimentary canal, the space between the two layers of mesoderm, is the coelom, and is filled in by coelomic fluid, which serves as hydrostatic skeleton also.
Excretion takes place by specialized structures called nephridia. These are ciliated organs present in each segment in the body cavity.
A well developed central nervous system is present in annelids. It comprises of a simple brain and a solid double, longitudinal, ventral nerve cord. Nerves arise in each segment from the nerve cord.
Annelids are the first group of invertebrates which have developed a closed circulatory system – a system in which a circulatory fluid called blood flows in a network of vessels known as blood vessels. It transports gases and nutrients.
The respiratory system is absent. The exchange of gases is by diffusion through the skin in to blood capillaries. The skin is kept moist by mucus, and coelomic fluid.
The body wall contains muscles which help in locomotion. The muscles are of two types:
- Circular Muscles: These are arranged along the circumference of the body.
- Longitudinal Muscles: These are arranged along the length of the body.
The locomotion is brought about by the interaction of muscles and hydrostatic skeleton. Contraction of circular muscle produces a pressure in the coelomic fluid that forces the body to elongate. Similarly contraction of longitudinal muscles produce a pressure in the coelomic fluid that would cause the body to widen. The organs of locomotion in annelids are chitinous chaetae or setae embedded in sacs (earthworm) or on parapodia present in the body wall (e.g., Nereis). Chaetae are absent in leech.
The common mode of reproduction is sexual. Most annelids (Earthworm, leech) are hermaphrodite. In some annelids (e.g., Nereis) the sexes are separate, the fertilization is external and a free swimming trochophore larva is produced during the life cycle.
Burrowing activity of Earthworms permits greater penetration of air into the soil, and improves drainage capacity of the soil. It also enables roots to grow downwards through the soil more easily. Mixing and churing of the soil is brought about when earth which contains inorganic particles is brought up to the surface from lower regions. Earthworm is perhaps most active segmented worm in churning the soil, therefore it is commonly termed as natural plough.
Phylum Annelida comprises:
- Class polychaeta
- Class Oligochaeta
- Class Hirudinea
Class Polychaeta
These have a distinct head region with eyes and structure known as palps and tentacles. Sexes are usually separate. The organs of locomotion are parapodia. They are mostly aquatic (marine). During development these give rise to a trochophore larva. Important examples are Nereis, Chaetopterus.
Class Oligochaeta
These animals have internal and external segmentation. Organs of locomotion are setae. Head region not prominent or distinct. They are hermaphrodite (bisexual). No larva formed during development e.g. lumbricus terrestris, pheretima posthuma and other earthworms. They may be terrestrial or aquatic.
Class Hirudinea
They have body with fixed number of segments. Each segment has additional circular rings or markings called annuli. They do not have organs of locomotion and move due to the contraction of their body and with the help of suckers. Mostly hermaphrodite and trochophore larva is formed during development. They are aquatic. No distinct head is present but leeches have chitinous jaws for making a puncture in the skin of the host. They also have an anticoagulant secretion which is passed into the wound to allow smooth flow of blood into its digestive system where it can be stored for a long time e.g. Hirudo medicinalis (medicinal leech).