Animal Body Plans, Excretion, and Nervous Systems Explained

Animal Body Plans and Organization

Animals (metazoans) are classified by their level of cellular organization, body symmetry, and embryonic development. This section outlines the main groups and their key features.

Levels of Cellular Organization

• Protozoa: single-celled organisms with all vital functions performed without embryonic development.
• Parazoa: multicellular animals (for example, sponges) with no true tissues; cells are loosely organized, and the only functional difference is between soma and germ cells.
• Mesozoa: multicellular animals with simple embryonic development, functional differences between morphs, and cellular stages (soma and germ).
• Eumetazoa: multicellular animals with embryonic development, morphological differentiation, and several organized tissues.

Eumetazoan Types

Diploblasty and Triploblasty

• Diploblastic: two germ layers, ectoderm and endoderm (for example, cnidarians).
• Triploblastic: three germ layers: ectoderm, mesoderm, and endoderm.

Body Cavities

• Acoelomate: no coelom; internal cavities are absent. Examples include flatworms, which are the most primitive.
• Pseudocoelomate: the blastocoel persists as a body cavity between mesoderm and endoderm, hosting the viscera (for example, nematodes).
• Coelomate: the mesoderm lines the ectoderm and the endoderm, forming a true coelom. Examples include molluscs and annelids.

Symmetry

• Spherical (homoaxonic): infinite symmetry axes, globular body, adapted to floating; for example, protozoans.
• Bilateral: a single plane of symmetry, with a sagittal axis running cephalo-caudal, ventral, and transverse. Associated with active movement.
• Radial (monoaxonic): a single oral-aboral axis. Found in sessile or floating animals with a passive, tubular body.
• Polarized: when the two ends of the body axis are different.

Integument

The integument (tegument) performs protective, insulating, respiratory, excretory, thermoregulatory, sensory, skeletal, and feeding functions.

Types of Integument

• Protozoa: no integument.
• Metazoa, arthropods: an outer layer called the epicuticle, with lipids and proteins; the procuticle is made of proteins.
• Invertebrates: a monostratified epithelium with glands; the cuticle may form.
• Vertebrates: the outer part (epidermis) is a multilayered epithelium, with a thick dermis of connective tissue containing collagen and blood vessels.
• Fish and amphibians: epidermal cells are alive in all layers. Glands of epidermal origin are located in the dermis.
• Reptiles, birds, and mammals: the surface consists of dead cells filled with keratin (the stratum corneum), above living basal cells of the dermis (the germinative layer).

Excretion

Nitrogenous Waste Products

• Ammonotelic: ammonia (NH₃) is removed directly by diffusion. It is highly toxic and soluble, so it requires a large volume of water.
• Ureotelic: ammonia is converted into urea, which is soluble, less toxic, and stored temporarily in solution.
• Uricotelic: ammonia is converted into uric acid, which is non-toxic, non-soluble, and crystallizes; it is expelled with feces.

Excretory Organs

• Nephridia: protonephridia are ducts that open to the exterior through nephridiopores, where waste is dispersed into the water. Metanephridia have a nephrostome and tubules; the tube leads to the exterior through a nephridiopore, and it absorbs useful substances.
• Malpighian tubules: tubes that lead to the digestive tract in insects; they remove NH₃ and convert it into uric acid crystals.
• Kidneys: organs made of functional units called nephrons, which form urine. Each nephron pours urine into the urethra.

Respiration

• Gills (branchiae): evaginations of the integument, permeable and without cuticle, with ciliated epithelium and a vascular network. They can be external or internal (housed in a protective chamber).
• Tracheae (terrestrial): invaginations of the cuticle forming air tubes without a circulatory system. They open to the exterior through spiracles.
• Lungs: invaginations of the body surface with membranes irrigated by a circulatory plexus that transports gases to the cells.

Nervous System

• Protozoa and Porifera: no nervous system.
• Cnidarians: a nerve net (nerve plexus).
• Most invertebrates: a bilateral model with ganglia and nerve cords.
• Vertebrates: the brain has three parts: forebrain, midbrain, and hindbrain.
• Mammals: the central nervous system (CNS) includes the brain and spinal cord; the peripheral nervous system (PNS) includes sensory and motor neurons, divided into somatic and autonomic (sympathetic and parasympathetic) systems.

Embryonic Development

• Holoblastic: the entire egg divides.
• Meroblastic: only part of the egg divides, typically in eggs with more yolk.
• Protostomes: the blastopore becomes the mouth; spiral cleavage; schizocoelic coelom formation.
• Deuterostomes: the blastopore becomes the anus; radial cleavage; enterocoelic coelom formation.

Representative Examples by Group

• Tapeworm (Moniezia expansa): class Cestoda, phylum Platyhelminthes.
• Liver fluke (Fasciola hepatica): class Trematoda, phylum Platyhelminthes.
• Scallop (Pecten maximus): class Bivalvia, phylum Mollusca.
• Garden snail (Helix aspersa): class Gastropoda, phylum Mollusca.
• Sponge (Sycon ciliatum): phylum Porifera.
• Portuguese man o' war (Physalia physalis): class Hydrozoa, phylum Cnidaria.
• Sea anemone (Anemonia sulcata): phylum Cnidaria.
• Common octopus (Octopus vulgaris): class Cephalopoda, phylum Mollusca.
• Earthworm (Lumbricus terrestris): class Oligochaeta, phylum Annelida.
• Medicinal leech (Hirudo medicinalis): class Hirudinea, phylum Annelida.