Mesozoa and Parazoa

• Specializations in metazoans are distributed over cells, tissues, organs, and organ systems
• Metazoan cells are less complex but more specialized for particular functions than protozoan cells

Multicellular Groups

• Multicellular organisms are divided into three groups: Mesozoa, Parazoa, and Eumetazoa
• Mesozoa and Parazoa are multicellular but lack germ layers
• Mesozoa and Parazoa have a cellular level of organization
• Mesozoans are entirely parasitic and have a complex reproductive cycle

Mesozoa

• Mesozoa is a small phylum
• Simple bodies consisting of 20–30 cells in two layers but lacking germ layers are characteristic of Mesozoa
• All known Mesozoa species are internal parasites of marine invertebrates
• Class Rhombozoa lives in the kidneys of cephalopods like octopuses and squids
• Class Orthonectida parasitizes a variety of invertebrates, such as brittle stars, bivalve molluscs, and polychaetes

Parazoa

• It is a subkingdom of animals including sponges
• An early branching event in the history of animals separated the sponges from the Metazoans

Eumetazoans

• Metazoan refers to multicellular animals
• Eumetazoans, that make up most of the Metazoa, evolved greater structural complexity by combining cells into greater units
• Cells of multicellular organisms are specialized for performing various functions
• With the exception of sponges, metazoan cells are arranged into tissues
• Tissues are necessary to produce organs and organ systems
• Tissues, organs, and organ systems enabled the evolution of large, multicellular bodies
• Most Metazoa have an additional level of complexity in which different organs operate together as organ systems

Animal Symmetry

• Symmetry refers to proportion balance in size and shape of parts on opposite sides of a median plane
• Asymmetrical animals have no pattern of symmetry, for example, the simplest animals like sponges
• Radial Symmetry, body parts are arranged around a central axis so that each part extends from the center
• Animals with Radial symmetry can be divided into halves by more than two planes passing through the longitudinal axis
• Radial animals are usually sessile, freely floating, or weakly swimming
• They have no front or back end, and interact with their environment in all directions
• This is advantageous for sessile or free-floating forms when snaring prey
• Cnidaria and Ctenophores are primarily Radiata phyla
• Bilateral Symmetry, the animal can be divided along a sagittal plane into two mirrored portions, right and left
• Bilateral animals are much better fitted for directional movement than radially symmetrical animals
• Bilateral symmetry is strongly associated with cephalization, the differentiation of a head
• Cephalization is always accompanied by differentiation along an anterior posterior axis, with the anterior end having feeding and sensory structures
• Bilateral symmetry and cephalization give several advantages to an animal
• They have more ways to move
• They have more ways of interacting with other organisms and their physical surroundings
• Animals that have heads are often active and mobile, moving head first through their environment
• Sensory organs are at the head end meaning the animal can test for food, danger, and mates as it enters new surroundings
• Sponges lack symmetry and Cnidarians exhibit radial symmetry
• All other animals have bilateral symmetry

Embryonic Development in Metazoans

• A fertilized animal egg divides to produce a solid ball of cells
• Cell migration results in a hollow ball called a blastula
• Blastula cells migrate inward producing a gastrula
• The opening of the gastrula is the blastopore
• The tube produced by the gastrula becomes the gut, or digestive tract of the mature animal

Protostome and Deuterostome Conditions

• In protostomes, the initial opening develops into the mouth, and an opening that develops later becomes the anus
• In deuterostomes, the initial opening develops into the anus, and an opening that develops later becomes the mouth
• Protostomes are a group of animals with a body cavity in which the blastopore is associated with the mouth
• Deuterostomes are a group of animals with a body cavity in which the blastopore is associated with the anus and a second opening is associated with the mouth
• Indeterminate cleavage is characteristic of deuterostomes
• Following the initial cell division, the fate of the resulting daughter cells is not determined, each has the potential to develop into an entire organism
• Determinate cleavage is characteristic of protostomes.
• After the initial cell division, the fate of the resulting daughter cells is determined, they can only develop into specific tissues, not the whole organisms
• Radial cleavage is characteristic of deuterostomes
• As the embryo undergoes cell division, and changes from a four-cell embryo to an eight-cell embryo, the cells divide such that each cell in the top four-cell plane is directly over one other cell in the bottom plane
• Spiral cleavage is characteristic of protostomes
• As the embryo undergoes cell division, and changes from a four-cell embryo to an eight-cell embryo, the cells divide at slight angles to one another, so that none of the four cells in one plane of the eight-cell stage is directly over a cell in the other plane

Germ Layers

• These are the three layers of tissues that become established during early embryonic development
• They give rise to the tissues of the body
• They consist of the ectoderm, mesoderm, and endoderm
• The Ectoderm forms from the outer layer of cells and gives rise to the skin and nervous system
• The Endoderm forms the lining of the gut and the organs derived from the gut
• The Mesoderm forms between the ectoderm and endoderm, becoming the muscles, connective tissues, skeleton, kidneys, circulatory and reproductive organs

Body Cavities

• A body cavity or coelom refers to the space located between an animal’s outer covering or epidermis, and the outer lining of the gut cavity, where internal organs develop
• The coelom is a body cavity formed between layers of mesoderm and in which the digestive tract and other internal organs are suspended
• It separates the gut and internal organs from the rest of the body
• Isolates the internal organs from body-wall movements
• Bathes the internal organs in a liquid through which nutrients and wastes can diffuse
• Triploblastic animals can be grouped in three categories according to the presence or absence of a body cavity or coelom, in addition to the digestive cavity: acoelomates, pseudocoelomates, and coelomates
• Acoelomates: Animals in which tissues derived from the three germ layers are packed together and there is no body cavity other than the digestive cavity such as Platyhelminthes
• Pseudocoelomates: Animals in which there is an additional cavity that develops between the endoderm and the mesoderm, this cavity is known as a pseudocoelom such as Nematoda, Rotifera, Nematomorpha, Gastrotricha and Acanthocephala
• Coelomates: Animals that are three-layered with a true coelom, a fluid-filled cavity that develops within the mesoderm
• Within the coelom, the digestive tract or gut, and other internal organs are suspended by the mesodermal mesenteries such as mollusks and annelids
• The coelom confers several advantages to the animal
• Reproductive and digestive organs can evolve more complex shapes and functions
• The gut tube and other organs are cushioned and thus better protected in a fluid-filled chamber
• Since a liquid cannot be compressed, the coelom can act as a hydroskeleton, providing support and rigidity for the soft animal
• The activities of a suspended gut can take place undisturbed by the activity or inactivity of the animal’s outer wall
• Enterocoelous development of the coelom occurs in deuterostomes
• The mesoderm and coelom initially develop as pouches off of the primitive digestive tract, the archenteron
• The mesoderm buds from the walls of the archenteron and hollows to become the coelomic cavities
• Schizocoelous development of the coelom occurs in protostomes
• The mesoderm and coelom initially develop from a solid block of mesoderm tissue that develops a split down the middle
• As the archenteron initially forms, the mesoderm splits to form the coelomic cavities
• Coeloms in some triploblastic animals have been subsequently lost
• Loss of coelom and reduction in body size is correlated.
• Coeloms are absent in diploblastic animals such as cnidarians

Gut Development

• The gut is the digestive tract
• It enables the animal to digest food outside of the cells or extracellular digestion
• In animals without a digestive tract, food items are brought into the cell for digestion by intracellular digestion
• A sac-like gut has one opening where food enters and leaves
• A complete gut has two openings, a mouth and an anus, and is a tube-within-a-tube

Summary of Evolutionary Trends in the Invertebrates

• Five major anatomical and physiological trends are revealed
• Away from radial symmetry towards bilateral symmetry
• Cephalization or the development of a head, with its sensory apparatus that detects environmental stimuli
• Away from a simple, sac-like body with a single opening at one end towards a more complex, elongated body containing a food-digesting tube, the gut, with openings at both ends
• Away from enclosure of the tube in solid tissue towards suspension of the tube in a fluid-filled space, the coelom, this cushioned the gut and helped the whole body from within, and allowed other internal organs to develop more complex forms
• Towards segmentation of the body and a development of a series of body units, each containing similar sets of muscles, blood vessels, nerves, and other structures, segmentation allowed animals to develop specialized body parts such as legs, wings, and antennae that perform very specific functions