Animal Anatomy: Body Plans and 3D Models of Every Animal Group

Animals are many-celled organisms whose cells have no walls. All of them are heterotrophs that must eat other organisms, most can move during at least part of their lives, and most are built from specialised tissues: epithelia that cover surfaces, connective tissue such as bone, muscle and nervous tissue. Biologists divide the kingdom into five clades: sponges (Porifera), Placozoa, Cnidaria, comb jellies (Ctenophora) and Bilateria, which holds every other animal. (OpenStax Biology 2e, 27.1 Features of the Animal Kingdom.)

The body plan is laid down in the embryo, and Hox genes, master control genes that encode transcription factors, help set it. Vertebrates have four or more sets of Hox genes, while invertebrates have one. (OpenStax Biology 2e, 27.1 Features of the Animal Kingdom.)

Nearly 97 percent of animal species are invertebrates, animals without a backbone, and perhaps millions of species have not yet been scientifically classified. (OpenStax Biology 2e, 28 Introduction (invertebrates).)

Estimates of that share depend on how species are counted. The Australian Museum puts invertebrates at well over 90 percent of animal species, with about 1.7 million invertebrate species documented so far and an estimated 5 to 30 million in total. (Australian Museum, Invertebrate biodiversity.)

Vertebrates belong to the chordates, which all have a notochord, a dorsal hollow nerve cord, pharyngeal slits and a post-anal tail at some stage of life. In vertebrates the notochord is replaced by the vertebral column that gives the group its name, and the nerve cord develops into the brain and spinal cord. With more than 62,000 living species, vertebrates are the largest group of chordates. (OpenStax Biology 2e, 29.1 Chordates.)

Arthropods dominate the invertebrates, with well over a million described species and an estimated 85 percent of all known species. They are coelomate animals with a tough exoskeleton made mainly of chitin and jointed appendages, which is what the name means: jointed feet. (OpenStax Biology 2e, 28.6 Superphylum Ecdysozoa: Arthropods.)

Sponges are asymmetrical. Jellyfish and sea anemones are radially symmetrical, with body parts arranged around a central axis, a plan best suited to animals that stay put or move little. Most animals are bilaterally symmetrical: one midsagittal plane divides them into left and right halves, they have a head and tail end (anterior and posterior) and a back and belly (dorsal and ventral), and bilateral symmetry allows streamlined, directional movement. (OpenStax Biology 2e, 27.2 Features Used to Classify Animals.)

Bilateral animals usually show cephalization, the gathering of an organised nervous system at the front end. Echinoderms are the famous exception that proves the rule: adult sea stars and sea urchins are radial, but their larvae start out bilateral, so echinoderms evolved from bilateral ancestors and are classified with them. (OpenStax Biology 2e, 27.2 Features Used to Classify Animals.)

Embryos build the body from germ layers. Radially symmetrical animals such as cnidarians are diploblasts, with only ectoderm and endoderm and a non-living layer between them; bilateral animals are usually triploblasts, with mesoderm as a third, middle layer. (OpenStax Biology 2e, 27.2 Features Used to Classify Animals.)

Triploblasts differ in their body cavity. Flatworms are acoelomates, with no cavity between gut and body wall. Roundworms (nematodes) are pseudocoelomates, with a cavity lined partly by mesoderm and partly by endoderm. Annelids, molluscs, arthropods, echinoderms and chordates, vertebrates included, are eucoelomates, with a true coelom lined entirely by mesoderm. (OpenStax Biology 2e, 27.2 Features Used to Classify Animals.)

A coelom cushions the organs it encloses and absorbs shock, lets them grow and move freely, gives room for gases and nutrients to diffuse, and houses the major arteries and veins. (OpenStax Biology 2e, 27.2 Features Used to Classify Animals.)

In annelids such as the earthworm, many internal and external features repeat in every segment, a pattern called metamerism. A membranous septum closes off each segment and divides the coelom into compartments, and each segment carries its own pair of excretory metanephridia. Metamerism lets an animal grow bigger by adding compartments and makes its movement more efficient. (OpenStax Biology 2e, 28.4 Superphylum Lophotrochozoa: Mollusks and Annelids.)

Arthropods fuse groups of segments into body regions called tagmata: head, thorax and abdomen in insects, or a cephalothorax and abdomen in other groups. An insect's head alone is the fused remains of six ancestral segments, and the jointed appendages, often one pair per segment, are specialised for sensing, feeding, walking, jumping, digging or swimming. (OpenStax Biology 2e, 28.6 Superphylum Ecdysozoa: Arthropods.)

Start with the body plan, then the systems. For any animal, ask four questions first: what symmetry it has, what body cavity, whether it is segmented, and whether it has a backbone. Then work through one organ system at a time and compare it with the same system in an animal you already know; the comparative anatomy guide does this for limbs, hearts, guts and lungs.

Use 3D models to learn where structures sit. Most models on this site can be rotated and taken apart layer by layer, and clicking a structure names it and explains what it does, which is the fastest way to learn positions before a practical exam.

Virtual and model-based dissection works. A 2021 systematic review of 50 studies found that humane teaching methods, such as models, simulations and virtual dissection, gave learning outcomes that were superior to harmful animal use in 30 percent of studies, equivalent in 60 percent and inferior in 10 percent. (Zemanova & Knight (2021), the educational efficacy of humane teaching methods, Animals 11: 114 (PMC).)

Each dissection follows the usual lab sequence step by step, from external anatomy to the internal organs, and highlights each structure on a 3D model as you reach it:

• Squid dissection
• Perch dissection
• Shark dissection
• Starfish dissection
• Clam dissection
• Cat dissection
• Frog dissection
• Pigeon dissection
• Fetal pig dissection
• Rat dissection
• Grasshopper dissection
• Earthworm dissection
• Crayfish dissection
• Cow eye dissection
• Sheep brain dissection
• Sheep heart dissection

The same 3D approach covers the human body, plants, cells and microbes: