Phylum Aschelminthes or Nementhelminths- The Round Worms

Gegenbaur (1859) proposed the term 'Nementhelminthes' . The phylum Nementhelminthes includes about 15,000 species.

GENERAL CHARACTERS

1. Habitat- Many roundworms live as parasites in plants and animals. They cause serious agricultural, veterinary and human health problems. Round worms are also free living and occur in freshwater, sea water and soil.
2. Body Form - They are called roundworms because they appear circular in cross section. They are unsegmented.
3. Body Wall- It consists of firm, non-living resistant cuticle, syncytial epidermis and muscle layer.
4. Body Cavity- The body cavity is called pseudocoelom because mesoderm is found as scattered pouches inbetween the ectoderm and endoderm.
5. Digestive tract- Alimentary canal is complete with muscular pharynx.
6. Skeletal, Respiratory and Circulatory systems are absent. Pseudocoelomic fluid pseudocoelom maintains body shape and forms hydroskeleton. Gaseous exchange is aerobic respiration occurs by diffusion through the body surface. The pseudocoelomic fluid transports materials.
7. Excretory system- The excretory system consists of gland cells. In Ascarias, 'H' shaped excretory system of canals and complicated 'giant cells' called 'renette cell' is present.
8. Nervous system- It consists of Circumpharyngeal ring that gives rise to nerves forward as well as backward.
9. Sexes- Sexes are separate (dioecious) Generally they show sexual dimorphism , often females are longer than males. Fertilization is internal.
10. Development- Development may be direct or indirect. During indirect development a larvae is present. Filariform larvae is present in Ancyclostoma, microfilaria larvae is present in Wuchereria and Rhabdiform larvae is present in Ascaris and enterobius.
    

Classification

Aschelminthes are classified into two classes 

Class 1. Aphasmidia: (i) Phasmids are absent    (ii) Amphids are of various types , e.g,. Trichinella ,Trichuris etc.

Class 2. Phasmidia: (i) phasmids are present near hind end of the body. (ii) Amphids are present near anterior end, e.g., Ascaris, Enterobius, Ancyclostoma, Wuchereria etc.

Key points 

• Dual excretory waste: Ascaris is both Ammonotelic and ureotelic.
• Monorchic: Presence of single testes, e.g., male Ascaris.
• Didelphic: Presence of two uteri, e.g, Female ascaris.

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Phylum-Ctenophora

• Ctenophores are commonly called comb-bearers.
• Ctenophora derived from two greek words Ktenes-  comb and phores- bearing.
• Also called comb-jellies and sea walnuts.

General characteristic  features of phylum- Ctenophora

1. Habitat- Ctenophores are exclusively marine and free swimming animals.
2. Body Symmetry- radially symmetrical.
3. Level of organization - Like Cniderians, they also exhibit tissue level of organization.
4. Germ layers- diploblastic . Outer ectoderm and inner endoderm are present with an undifferentiated layer mesoglea in between them. Mesoglea gives jelly like appearance to these animals.
5. Body form- They have a transparent body. Body shape varies from flat to oval. External surface of the body bears eight median comb plates. These comb plates bears cilia which are fused to make these plates ciliated. The ciliated comb plates help in locomotion. 
6. Tentacles may be present or absent. When present the no. of tentacles is 2. They are solid and possess adhesive cells called colloblasts (lasso cells). Digestion is both extracellular and intracellular . Bioluminescence is well marked in ctenophores.
7. Digestion- both intracellular and extracellular. The food is captured by tentacles and directed to the gastro-vascular cavity of body. First the digestion takes place in the cavity then inside the food vacuoles.
8. Gastrovascular cavity is branched and opens to the exterior of stomodaeum.
9. Presence of a special sense organ 'Statocyst' at the opposite end of the mouth is the characterstic of the members of this phylum.
10. They are diploblastic animals but the mesoglea differs from cnideria; it contains amoebocytes and smooth muscle cells and is comparable to a loose layer of cells. From this viewpoint, Ctenophores may be considered as triploblastic.
11. Reproduction- Ctenophores are hermaphrodite. Sperms and ova are produced by the same individuals. Asexual reproduction is absent. Reproduction only takes place by sexual means.
12. Fertilisation- The fertilisation is external as fusion of gametes takes place outside the body of animal, in water.
13. Development - After fertilisation , zygote develops into a free swimming  cydipped larva which later develops into adult . Hence the development is indirect.
14. Bioluminescence - Bios means living and luminesence means light . It is the property of production and emission of light by living organisms and hence shine in the dark background. 
15. Examples- Pleurobrachia( sea gooseberry), Ctenoplana, Hormiphora (sea walnut), Cestum(venous girdle), Biröe. 

 Ctenoplana is a creeping animal. It lives on the surface of water of sea.
Related links  Phylum Porifera
                       Phylum Cnideria

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Phylum Cnideria ( Coenlentrata )

 Cniderians were proved as animals first time by Peysonnel and Trembley.
Leukart used the term coelentrata and Hatscheck used the term cnideria.
The phylum Cnideria includes about 9,000 species.

General characters
1. Habitat- All are aquatic and mostly marine , except some like hydra, are fresh water.
2. Body Form- Body form varies considerably. Many colonial - like Obelia are trimorphic , having three kinds of zooids-polyps, blastocysts and medusae. Occurrance of more than one type of individuals in their colonies performing different functions is called polymorhism.
3. Symmetry- shows radial symmetry.
4. Germ layers- diploblastic animals., i.e.,derived only from two germ layers endoderm and ectoderm.
5. Level of organization- first multicellular animals from evolution point of view showing tissue level of organization.
6. Body wall- consists of two layers of cells ; outer- epidermis and inner gastrodermis. There is a non- cellular gelatinous layer called mesoglea, between the epidermis and gastrodermis.

1. Epidermis- epidermis consists of following cells .(i) Epitheliomuscular cells - provides protection and acts as muscles. (ii) Cnidoblasts ( stinging cells ) - Cnidoblast has nematocyst ( the stinging organ ) .The nematocyst consists of capsule, shaft and thread tube. These are used for defence and offense.  (iii) Interstitial cells - Reserve cells and called totipotent cells which can be converted into any type of cells ( iv) Nerve cells - form primitive nervous system. (v) Sensory cells- sensory in function.
2. Gastrodermis- The gastrodermis comprises of (i)Nutritive muscular or digestive cells- Inracellular digestion takes place inside these cells and these also acts as muscles; (ii) Gland cells- secretes digestive enzymes for extracellular digestion; (iii) Interstitial cells (iv) Nerve cells  and (v) Sensory cells. Functions of interstitial, nerve and sensory cells are similar to that of epidermis cells.

7.Digestive tract- have a central gastrovascular cavity ( coelenteron ) with a mouth, which also acts as anus . 
8. Digestion - both intracellular and extracellular type of digestion present.
9. Respiration and Excretion are carried out through body surface by diffusion. Main excretory product is ammonia.
10. Nervous system- primitive type of nervous system present which consists of a network of nerve cells and their processes. Statocyst is a sense organ for balance which is first time developed in Cnideria.
11. Skeleton - In some Coelentrates body is supported by horny or calcareous exoskeleton or endoskeleton.
12.Reproduction- both by asexual ( budding) and sexual methods. Both gonads and buds arise from the interstitial cells The power of regeneration is also developed.
13. Development- cleavage is holoblastic . Direct or indirect development is found . In obelia planula larvae is present. However in aurelia planula, scyphistoma and ephyra larvae are found.
14. Metagenesis - In obelia , polyps reproduce medusae asexually and medusae form the polyps sexually. Such alternation of asexual and sexual phases in the life cycle of obelia is called metagenesis. It shouldn`t be confused with alternation of generation as found in plants where one phase is haploid and other is diploid. Here  both phases are diploid.

Distinctive features-
presence of cnidoblast for defense and offense , network of nerve cells acting as primitive nervous system.

 Advancement over porifera-  tissue level of organization, digestive tract, nerve and sensory cells.

Classification

Chiefly on the basis of the dominance of medusoid or polyploid phase in the life cycle, the phylum Cnideria is divided in three major classes- 
Class 1 - Hydrozoa-

• both marine and fresh water.
• Solitary or colonial.
• Either only polyps are found or polyps and medusae are present exhibiting polymorphism. 
• Medusae are always sexual forms and bear true velum which is marked by a narrow shelf- like ridge on the inner border of the circular margin of the medusae.Such a medusae is called as Craspedote.
• Many hydrozoa exhibit alternation of generations (-metagenesis) 
• gonads are usually ectodermal in origin and sex cells are discharged externally ,
• ex.-Hydra ( fresh water polyp ), Obelia(sea-fur) , Physalia( portuguese man of water ).

Class 2 - Schyphozoa-

• Exclusively marine.
• solitary and represented by a bell shaped medusae which are without velum. They are commonly called jelly fishes.
• Polyp form is reduced or absent.
• mesoglea is much enlarged and gelatinous.
• gonads are endodermal in origin. The sex cells are discharged into the gastrovascular cavity.
• Examples,- Aurelia(jelly fish) , Rhizostoma. 

Class 3- Anthozoa- 

• exclusively marine .
• attach themselves with the rocks and other submerged animals.
• They are solitary or colonial.
• Only polyp form is present. Medusae form is absent.
• The gastrovascular cavity is divided into compartments by complete or incomplete mesenteries. 
• Gonads are endodermal and develop into mesenteries . Sex cells are discharged into the gastrovascular cavity.
• Examples- Gorgonia ( sea fan ), Metridium ( sea anemone ) , Alcyonium ( dead man`s finger ), Fungia ( mushroom coral ) , Penntula ( sea pan or sea feather ), Corallium ( red coral ), Astraea ( star coral ), Madrepora (stag-horn coral), Tubipora(organ pipecoral), Heliopora( blue-coral) ,Meandrina ( brain coral) .

Polymorphism- Occurrence of the same species in more than one type of individuals , which differ in the form and function from one another, is known as polymorphism. This ensures an efficient division of labour between the several individuals which often get united to form colonies, e.g., polyp and medusae are examples of such organisms which differ in structure and function and the cnidarian possessing both of them are said to be exhibiting polymorhism., e.g., Obelia, Physalia etc.

1. Physalia (Portuguese man of war) : Physalia exhibit a remarkable example of polymorphism and division of labour. A gas gland is present inside pneumatophore secretes a gas which helps the animal to float over the water surface.
2. Pennatula (Sea pan) :It looks like a quill( a type of feather or pan ). It is carnivorous.
   
3. Gorgonia(sea fan) : All the branches form a hand held fan like structure .So, Gorgonia is commonly called sea fan.
   

Phylum Porifera

• Porifera means pore bearing ( Porous-pore ; ferre - bearing )
• phylum porifera genrally consists sponges.
• Sponges were first recognised as animals by John Ellis(1765).
• In 1825 Robert Grant gave the name Porifera.
• About 5000 species of sponges are known till now.

General characters

1. Habitat- Aquatic, mostly marine ,rarely fresh water (e.g., spongilla) , solitary or colonial, sessile . Sponges are mainly found in warm water.
2. Body form- body is porous. pores are of two types : inhalent pores- ostia and exhalent pores- Oscula.
3. Symmetry- Mostly asymmetrical. Some shows radial symmetry.
4. Germ layers- Sponges are first multicellular diploblastic animals i.e., derived from two germ layers ectoderm and endoderm.
5. Level of organisation - have cellular level of organisation.
6. Body wall- Body wall of common sponge consists of three layers.  

      A) Pinacoderm( dermal layer) -  Outer cellular layer  consisting pinacocytes and porocytes.

       B) Choanoderm( gastral layer) - Inner cellular layer which  consists of highly specialised cells called choanocytes or collar cells.

       C) Mesohyl layer ( Mesenchyme)- Non-cellular layer found inbetween pinacoderm and choanoderm. It has fine dispersed spongin fibres, spicules and amoebocytes.

7. Canal System- Three types of canal system are found in sponges: (i) Ascanoid canal system- simplest type found in Leucosolenia and few other sponges. (ii) Syconoid canal system- More complex then Ascon type. found in Sycon and some other sponges. (iii) Leuconoid canal system- most complex canal system found in spongilla.

Central body cavity of a sponge is called spongocoel or paragastric cavity.

The canal system helps the sponge in nutrition, respiration , exretion and reproduction.

8. Skeleton- Almost all sponges possess an internal skeletal system. It may consist of calcereous spicules or fine spongin fibres or of both , located in the mesohyl layer.

9. Digestion- intacellular and takes place inside food vacuoles.

10. Circulation- Distribution of food from the ingesting cells to the others is brought about by the wandering amoebocytes of mesohyl layer.

11. Respiration-exchange of gases by diffusion through the plasma membranes of the cells .

12. Excretion- By diffusion through the plasma membrane of cells. Chief excretory product is Ammonia.

13. Reproduction- Both sexual and asexual type of reproduction occurs. Asexual reproduction takes place by budding and gemmules. Sponges are hermaphrodite. Fertilization is internal.

14. Development- Zygote undergoes holoblastic cleavage. Development includes a free swimming larva, the amphiblastula( in sycon ) or parenchymula ( In Leucosolenia ) for dispersal of the species.

Unique features of phylum porifera-

Presence of Ostia and oscula , canal system and skeleton system made up of spongin fibres and spicules.

Classification
On the basis of skeleton, phylum Porifera is divided into three classes-
Class 1- Calcarea ( calcis= lime ) -The skeleton is of calcerous spicules.Examples;- Leucosolenia, sycon, grantia.
Class 2 - Hexactinellida ( hexa - six ). The skeleton is of siliceous spicules which have six rays. Ex.- euplectella, hyalonema.
Class 3- Demospongiae ( demas- frame ) - The skeleton is of spongin fibres with siliceous spicules or may be absent. Ex;- euspongia , spongilla, cliona, chalina.

•  Leucosolenia- simplest colonial sponge consisting of a number of horizontal and vertical tubes. The development is with a larva, the parenchymula.
• Sycon ( Scypha)- development is with a larva, the amphiblastula.
• Euplectella( The Venus flower basket) - found in deep sea water. They are abundent near the Phillipine Island and West Indies. Its skeleton is costly marriage gift in Japan as it is thought to be symbol of union of wife and husband.
• Hyalonema( The glass rope sponge) - It is fixed in the mud by a root tuft of long, twisted spicules. The upper surface has the gastral cone which bears opening of the excurrent canals. 
• Euspongia - the bath sponge - surface is raised into small projections, called connules. Depressions between connules bears small pores , the dermal ostia. Its skeleton is used for bathing, washing automobiles, cleaning furniture and window panes. It is used for applying cosmetics, mopping etc.
• Spongilla- fresh water sponge-Spongilla a common, widely distributed fresh water sponge shows various shades of green colour because of the symbiotic algae present inside.
• Cliona - the boring sponge - The sponge can penetrate the rocks and break them into pieces  . The sponge can also bore through the oyester shell and, therefore, harmful for pearl oyester industry.
• Chalina - the deadman`s fingers or the Meramaid`s gloves- It is shaped like a hand with several fingers perforated with oscula hence called so.

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