Biology of lower Animals, Biology tutorial

System of Classification:

Aristotle was the first to try logical system of classification. He separated all living organisms in two broad kingdoms, that is, plantae and Animalia. Kingdom plantae have mostly immobile forms, while kingdom Animalia has mobile forms.

Kingdom Animalia:

This kingdom can be divided into two sub-kingdoms namely;

1) Sub Kingdom protozoa:

This sub - kingdom has only one phylum - protozoa

2) Metazoan (acellular) (multicellular):

This sub - kingdom has many phyla.

i) Porifera or sponges

ii) Coelenterata - e.g. hydra, obelia

iii) Platyhelminthes - flatworms e.g. planaria, fasciola and taenia.

iv) Nematoda - rounding e.g. Ascaris, hook-worm

v) Annelida - e.g. Lumbricus, nereis, leech

vi) Arthropoda - insects, flies, spider, centipedes

vii) Mollusa - e.g. starfish

viii) Chordata - Amphioxus, fish, rats, man etc.

Phylum protozoans:

General characteristics of protozoans:

1) Protozoan is eukaryotic acellular organisms in which body is not divided in cells or tissues.

2) Great majority of protozoans are microscopic they range in size from one micron (1m) as in case of planktonic micromonas to few millimeters such as some Amoeba species and cilliates.

3) Most protozoans happen as solitary individuals but there are several colonial forms like volvox.

4) Protozoans are found wherever life exists and is fluid medium or moist medium.

5) Reproduction is universally asexual by mitotic division by budding, fission and cyst formation. Sexual reproduction by conjugation or zygote formation (syngamy) is found in few species.

6) Different means of locomotion, using pseudopodia, flagella and cilia and direct movement have evolved in the group.

Sub - phylum:


Phylum protozoa is separated in four classes

1) Rhizopoda or sarcodina (Amoeba group):

  • Amoeboid protozoans are differentiated by presence of flowing extension of their body called as pseudopodia. These tubular cytoplasmic projections are utilized as locomotory organelles.
  • Group comprises familiar Amoeba and other marine, fresh water and terrestrial taxa. Amoeboid form may be the result of retention of ancestral protistan condition in some species.
  • The body is simple and not very differentiated. Amoeboid protozoans are either asymmetrical or show spherical symmetry and may be the simplest of protozoans.

There are 4 groups of amoeboid protozoans: amoebae, foraminiferans, helizoans and radiolarians. For these amoebae and foraminiferans belong to superclass Rhizopoda whereas radolarians and helizoans belong to superclass actinopoda, as they have axopods. Reproduction is by asexual method only and it is by binary transverse fission. Multiple fission may take place during encystment stage.

i) Amoebae:

It may be naked or enclosed in tests or shells. Marine, freshwater and parasitic naked amoebae have large usually tubular lobopodia or fine straplike filipodia which are used for locomotion and feeding. Shelled amoebae are found in sea, freshwater and soil. They are enclosed by shell composed of secreted organic material or foreign mineral matter cemented together. Large aperture in shell allows protusion of lobopodia or filipodia.

ii) Foraminiferans:

They are largely benthic marine species. They contain multichambered calcareous tests or shells having many pores, therefore name foraminifera or pore bearer. Single large opening permits cytoplasm to protrude out.

iii) Radiolarins:

They are completely marine planktonic species with spherical bodies and radiating pseudopodia called as axopods. Spherical body is separated in inner and outer parts. Inner regions have one to many nuclei and are surrounded by the central capsule with membranous wall. This is distinguishing characteristic of radiolarians. Capsule membrane is perforated with openings continuous with cytoplasm is known as calymma. Their skeleton is created by silicon dioxide or strontium sulphate organized in form of lattice spheres or radiating spines.

iv) Heliozoans:

They are spherical protozoans which occur in sea or in still bodies of fresh water. They are primarily situated in bottom debris. Fine needle - life pseudopodia radiate from surface of body. These are called as axopodia. Each axopod has central axial rod covered by the moving cytoplasm. Body of heliozoans comprises of outer vacuolated ectoplasmic cortex and inner dense medulla. The medulla contains a dense cytoplasm, one to several nuclei and bases of axial rods.

2) Mastigophora (or flagelleta):

1. These are protozoans which move by means of one or more flagella that are whip - like protoplasmic projection.

2. Reproduction is asexual by longitudinal binary fission as in euglena or multiple fission as in trypanosoma. In sexual reproduction isogametes are generated.

3. They are usually separated into two groups on the basis of nutrition:

a) Phytoflagellates are autotrophs:

That possesses chlorophyll or other related pigments and stock up food as fats, oils and starches (other than glycogen). They are free - living and assigned frequently to algal phyla. Instances are gonium, Euglena, volvox, chlamydomonas, pandorina, peranema, dinoflagellates.

b) Zooflagellates:

Are heterotrophs which are free-living, commensals or symbiotic or parasitic in other animals. A number of species like Trichonympha and myxotricha live within gut of termites and digest cellulose digest for themselves. Anterior end of these large, complex flagellates (at times over 30mm long) is covered with the elaborate pellicle and hundreds of flagella, but posterior end extends pseudopods and ingests bits of wood.

The flagellate is able to generate cellulose digesting enzymes, but insect host can't; and thus, depends on carbohydrates released by symbionts. Every time insect molts, it loses lining of hindgut and all its symbionts. If it is not capable to obtain new ones, it will starve to death, although it continues to feed normally, for it cannot digest the wood. Lashing movements allow flagellate to swim about, and its own flagella help in steering. Cells are not firm, therefore they can bend easily.

3) Ciliophora (the ciliates):

Ciliates are the largest and most homogenous groups of protozoans with more than 7200 species found in fresh and marine waters, and water film of soil. Approximately a third of ciliates are ecto - or endoparasites or commensals. Classic example of this group is flipper - shaped paramecium; other familiar examples are Didinium, vorticella, Balantidim, stentor.

All ciliates have cilia as their locomotive organelles; these are short, hair -like protoplasmic projections that are utilized for producing a extremely rapid movement.

Body is firm, being surrounded in firm but flexible outer covering known as pellicle.

Cytoplasm has clear distinction in to ectoplasm and endoplasm, but unlike members of Rhizopoda, there is a high degree of separation of body in definite organelles. Generally there are two kinds of nuclei:

i) A large meganucleus for vegetatives activity and

ii) A small micronucleus concerned with reproduction.

Asexual reproduction is through transverse binary fission. There is also sexual reproduction known as conjugation.

4) Sporozoa:

There are approx 4000 species of protozoa which have spore - forming stages in their life cycles. They are all parasitic animals. They do not have locomotive organelles (no pseudopodia, flagella nor cilia). Reproduction is through uninucleate bodies known as spores. Reproduction by asexual method is through multiple fission and sexual reproduction is by conjugation. Sporozoans are haploid except for zygote. Zygote feels meiosis which results in the infective spore - like stage sporozoite which, through multiple fission produces more sporozoites. They invade host and become feeding trophozoites. In some sporozoans trophozoites, by multiple fission called as schizogony produces merozoites. Each merozoite suffers multiple fission to generate more merozoites which ultimately undergoes gamogony or multiple fissions to create gametes which fuse to form zygote.

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