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Running head: KINGDOM ANIMALIA

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Kingdom Animalia
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Introduction

An animal can be defined as any organism of the kingdom Metazoa or Animalia that is
multicellular, lacks a cell wall, are heterotrophic and eukaryotic. Sponges are regarded as
animals because they are of the Phylum Porifera, heterotrophic, multicellular, lack cell wall and
reproduce by both asexually by budding and sexually method as they produce sperms even
though the lack gonads. Choanocytes produce sperms and form spermatic cysts whereas the
transformation of archeocytes form eggs. Unlike other organisms in the kingdom Metazoa,
sponges lack a body symmetry, organs, true tissues and they are sessile while most animals are
motile. Animals feed by ingestion while sponges feed by filter-feeding whereby they draw water
through tiny holes known as incurrent pores. Unlike plant cells that have rigid cell walls,
animals’ cells lack cell walls and their bodies have cells organized into tissues where each tissue
has as specific function. Tissues are further organized into specialized organs. The majority of
animals are diploid implying that adult cells have two copies of genetic material. The
development of these organisms is described by characteristic phases beginning with zygote then
embryo and lastly the fetus.
The kingdom Animalia is classified into 36 sub-divisions called the phyla whereby each
sub-division has similar functional and structural functions that separates it from other phyla.
Under the traditional biological methodology, there are nine phyla that are recognized. They
include phylum Chordata, phylum Porifera, phylum Annelida, phylum Aschelmeinthes, phylum
Platyhelminthes, phylum Coelentrata, phylum Echinodermata, phylum Mollusca, and phylum
Arthropoda. All these phyla have general characteristics such as the cell structure, body
symmetry whereby the majority of animals are symmetrically bilateral while primitive ones are
asymmetrical. The echinoderms and cnidarians are radially symmetrical.

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Phylum Porifera

The phylum Porifera consists of primitive marine organisms that lack muscles cells and
organs. There are roughly 8,000 species that exist and examples are Spongilla, Euspongia, and
Sycon. They are generally sponges and possess the described features. They have varying sizes
ranging from one centimeter to fifteen centimeters with bodies composed of loosely organized
cells. In terms of body symmetry, they are radially symmetrical or asymmetrical. Their feeding
system consists of a spongocoel or central cavity that is alienated into a series of branching
chamber for the circulation of water so that they can feed. Their body wall has many pores
known as osculum and ostia. The skeleton is made of spicules. Poriferans are considered
hermaphrodites, reproducing by both sexual and asexual means of production. Asexual
production is through budding while sexual method of production takes place through the fusion
of sperm cells and eggs. They have three types of cells known as choanocytes, pinacoytes, and
mesenchyma cells.
Types of Cells, Skeletons and Body Wall
Since sponges are the majority, they have simple bodies and are more than clusters of
free cells. Having specialized cells, phylum Porifera has different types of cells described below.
Pinacocytes
These are flat cells that are thin walled and line the exterior surface of the sponge. They
are marginally contractile and this contraction changes the sponge shape. Pinococytes develops
contractile porocytes that are tube-like controlling the circulation of water. These porocytes also
function as pathways for entrance of water in the body wall.
Mesohyl

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Originally known as mesenchyma cells, this is a jelly-like coating with amoeboid cells
below the pinacocytes. They are specialized cells, moving freely and perform specific functions
such as storage and transportation of food, skeletal elements, reproduction, and secreting forming
contracting rings in the overtures in the sponge wall.
Choanocytes
These are collar cells that are found below the mesohyl and create the lining of the inner
cavity. They are flagellated cells that have a ring of micro...