Introduction to Mesozoa:

The term Mesozoa comes from the Greek language 'mesos' means middle and 'zoon' means 'animal'. The term comes from the fact that when they were first explained, they were thought to be some kind of 'missing link' among the protists and animals. That is comprehensible as mesozoans are tiny, ciliated, worm-like animals; a Mesozoa looks similar to nothing so much as a few dozen ciliate protists clumped altogether in semblance of an extremely simple animal.

Most of the mesozoans live as parasites in the marine invertebrates. Such creatures are very simple animals having no respiratory, nervous, circulatory or digestive systems. However, a Mesozoa consists of no organs at all. A typical Mesozoa is merely a few millimeters long, having just a few dozen cells at most. (Some encompass just 20 body cells when mature.) Only around 100 species of mesozoans are recognized.

General features of Mesozoa:

a) Bilaterally symmetrical. 

b) Has no tissues or organs.

c) No nervous system.

d) Body has no internal cavity.

e) Body encompasses no digestive tract (gut).

f) Has several cells build up within other cells.

g) Reproduction is quite complicated comprising both asexual and sexual features.

General categorization of Mesozoa:

The two major Mesozoa groups are the Rhombozoa (Dicyemid) and the Orthonectida. The other groups at times comprised in the Mesozoa are the Placozoa and the Monoblastozoa.

Rhombozoans:

Rhombozoans, at times termed as Dicyemida in some older texts are parasites of cephalopods (that is, Octopus and Squid); they live in the animals kidneys. Rhombozoans encompass a more complex life cycle. Their fundamental body plan is a long thin central cell, termed as an axial or tube cell, enclosed by a coat of smaller ciliated cells that are arranged spirally about the axial cell.

Orthonectida:

The Orthonectida are parasites on a broad range of marine invertebrates comprising Platyhelminthes, Echinodermata, Mollusca and Annelida. Throughout the sexual phase they are gonochoristic (female and male). 

Introduction to Parazoa:

The Parazoa are an inherited subkingdom of animals, literally interpreted as 'beside the animals'. Parazoans distinct from their choanoflagellate ancestors in that they are macroscopic and encompass differentiated cells, however dissimilar 'true animals' (Eumetazoa), they don't encompass tissues. 

a) The only surviving Parazoans are the sponges that fit in to the phylum porifera. Some comprise Placozoa, a phylum which comprises of only one species, Trichoplax adhaerens, in the division, however they are as well at times placed in the subkingdom Agntozoa. 

b) Parazoa is asymmetrical (that is, they display no symmetry); all other animals will display some kind of symmetry. There are presently 5000 species, 150 of which are fresh-water. Larvae are planktonic and adults are sessile.

General features of Parazoa:

a) Phylum porifera.

b) Deficient in true tissues and organs.

c) Deficient in symmetry.

d) Multicellular.

e) Wholly aquatic, both marine and fresh-water

Introduction to Metazoa:

Metazoans have organ system of body organization in which tissues build up into organs to carry out various functions in the body. Coelenterates are radically symmetrical diploblastic Metazoa while others from helminths onwards are bilaterally symmetrical and triploblastic. There are three theories to describe the origin of Metazoa.

1) Colonial theory: This theory was stated by Butschli, Lankester and Haeckel (1874) and Hyman (1940), which states that metazoans developed from some colonial flagellate ancestor identical to Proterospongia or Volvox. Several protozoans like Gonium, Synura and Pandorina form morula-like solid colonies which superficially look like metazoans.

2) Syncytial theory: This theory was stated by Hanson (1958) and Hadze (1963) and proposes that multinucleate ciliates should have been the ancestors of the metazoans. Ciliates possess high grade of organelle growth and body organization among the protozoans. By partitioning the multinucleated cytoplasm having cell membranes the ciliate body could have become multicellular and given augment to Metazoa.

3) Polyphyletic theory: The theory was stated by Greenberg (1959) and Preston (1967) and hypothesizes that metazoans originated from various kinds of protozoan ancestors in various lineages by all the procedures stated in the above theories.

General features of Metazoa:

Animals encompass quite a few features that set them apart from other living things. Animals are eukaryotic and generally multicellular that separates them from bacteria and most of the protists. They are heterotrophic, usually digesting food in an internal chamber that separates them from algae and plants. They are as well differentiated from algae, plants and fungi by lacking the cell walls.

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