Animals are eukaryotic, multicellular, heterotrophic opisthokonts. Almost all animals (with the exception of Poriferans and Placozoans) have specialized tissue types. Almost all animals display aerobic respiration and the ability to move spontaneously (during at least part of the life cycle). Nearly all animals have Hox genes, which code for transcription factors that determine an animal’s body plan.
Animals, with few exceptions, have a common set of embryonic stages. The blastula and gastrula stages are unique to animals. During blastulation, the embryo is converted from a ball of cells to a hollow sphere of cells. This sets the stage for gastrulation, in which some cells migrate to the inside of the blastula. This cell migration results in the archenteron, which gives rise to the gut. The opening to the archenteron, the blastopore, will develop into the mouth or anus. Gastrulation also results in a two- or three-layer embryo. Diploblasticanimals have two germ layers, endoderm and ectoderm, while triploblastic animals also have mesoderm, which is specified between the inner and outer germ layers.
Image Credit: YassineMrabet, CC BY-SA 3.0
Each germ layer gives rise to specific tissues. Many of these are unique to animals. For example, muscle tissue and nervous tissue are hallmarks of animals. These allow animals to rapidly and precisely respond to sensory information, which is key to their heterotrophic lifestyle.
There are over 1.5 million described species of animals. About 1 million of these are insects. However, this is almost certainly a gross undersampling of total animal species, which has been estimated to be from 3 to 30 million or more. We will consider three major clades of animals, each of which contains multiple phyla, plus two phyla of “basal animals”.
The phylogenetic tree shown is the leading hypothesis for the evolutionary history of major animal groups. Basal animal phyla include poriferans, cnidarians, ctenophores, and placozoans (not shown on tree). Note that this is not a monophyletic group, just an informal designation. All other animals are bilaterians, meaning that they display bilateral symmetry. Bilaterians include the deuterostomes and protostomes. These are distinguished by several features of embryonic development, and are named for whether the blastopore becomes the mouth or the anus. Deuterostomes (anus first) include chordates, hemichordates, and echinoderms. Protostomes (mouth first) are further divided into two groups, the ecdysozoans and the lophotrochozoans. The ecdysozoans are animals that grow by ecdysis, a specific kind of molting, and include the arthropods, nematodes, onycophorans, tardigrades, and a handful of other phyla. The lophotrochozoans are the largest group by number of phyla, and include the annelids, platyhelminths, molluscs, and a number of other phyla.
Basal Animals: Poriferans and Cnidarians
Along with ctenophores and placozoans, sponges and cnidarians do not fit into one of the major clades of bilaterian animals. They are sometimes referred to as basal animals, reflecting their phylogenetic position at the ‘base’ of the animal tree. Unlike the protostomes and deuterostomes, they do not have three embryonic tissue layers or bilateral symmetry.
Poriferans
Sponges, or poriferans, are almost unique in being animals that do not possess true tissue. However, they do have cell types that perform specialized functions. The reason that we do not call these tissues is that sponge embryos do not undergo gastrulation during development. Therefore, the cell types are not homologous to the germ layers – endoderm, ectoderm, and mesoderm – of other animals.
Sponge cell types include the pinacocytes, choanocytes, sclerocytes, amoeboctyes, and others. The pinacocytes form the outer wall of the body, a protective layer dotted with pores. Choanocytes line the inner cavities of the sponge. They are flagellated and beat their flagella produce a current. Water enters through ostia, or pores in the outer body wall, and exits through the osculum (or oscula, if the sponge has multiple excurrent openings). Between the outer and inner layer is an acellular material called the mesohyl. It is inhabited by cells such as sclerocytes, which secrete calcium carbonate or silica to make spicules, and the amoebocytes, which move through the mesohyl to deliver food to other cells. The amoebocytes also function as the stem cells, and can differentiate into other cell types.
Sponges obtain food by phagocytosis of bacteria and other small particles. Digestion is therefore intracellular, unlike most animals. Sponges can reproduce sexually or asexually. Sponges are monoecious, but often display sequential hermaphroditism, meaning that they do not produce eggs and sperm at the same time. This prevents self-fertilization.
There are over 8,000 described species of sponges, currently arranged within four classes. Sponges are known in the fossil record as early as 580 mya, from the Doushantuo Formation. Placement of sponges in the animal phylogeny has been controversial for years. However, a recent phylogenetic analysis that used 1,719 genes in 97 species[1] supports the monophyly of sponges, and their position as a sister group to all other animals.
Cnidarians
Cnidarians are diploblastic animals with radial symmetry. Cnidarians exist in one of two body plans – polyp or medusa – and some cnidarians alternate between these during their life cycle. Both body types are lined on the outside by epidermis (which develops from the ectoderm of the embryo) and on the inside by gastrodermis (which develops from endoderm). Between is an acellular jelly-like layer called mesoglea.
Cnidarians have a distinctive cell types specific to the phylum. Cnidocytes are the stinging cells that cnidarians use to capture prey and defend themselves from predators. The cnidocytes are located around the mouth/anus and on the tentacles, and discharge specialized organelles, called nematocysts. The nematocyst is explosively discharged from a cnidocyte when the cell detects touch. Nematocysts are discharged at accelerations of over 40,000g (and possibly more than an order of magnitude above this).[2] Cnidarians have a simple nervous system, including sensory neurons, interneurons, and motor neurons, arranged in a nerve net that is distributed throughout the body. They have a single opening to the digestive cavity, which serves as both mouth and anus.
There are over 10,000 described cnidarian species, which are divided into four classes.
- Class Hydrozoa – siphonophores, Portuguese man o’ war, Hydra, Obelia, etc.
- Class Scyphozoa – jellies/jellyfish
- Class Cubozoa – box jellies
- Class Anthozoa – corals, sea fans, sea pens, sea anemones
Deuterostomes: Chordates, Hemichordates, and Echinoderms
Chordates, Hemichordates, and Echinoderms are all deuterostomes. Clade Deuterostomia differs from other bilaterian animals in several features of embryonic development. The blastopore develops into the anus, the mesoderm buds from the endoderm surrounding the archenteron, and cleavage is indeterminate. Indeterminate cleavage means that cells of the blastula have not yet been specified to have a particular fate. This is what makes identical twins in humans possible, for instance. Each cell at the two-cell stage can develop into a complete organism. This is not the case of protostome embryos, which display determinate cleavage.
Chordates
Chordates are united as a clade by four key features – a dorsal, hollow nerve cord, a notochord, a post-anal tail, and pharyngeal slits. Not all chordates possess these features throughout their life – often some or all of these features may be present only during the embryonic stage. Most chordates are vertebrates, including familiar groups such as fish, amphibians, reptiles (including birds), and mammals. However, there are also invertebrate chordates.
Chordate clades are listed below. Taxonomic rank is fairly meaningless at this level, and is only provided to show which clades nest inside which. Although chordates are relatively well studied, this taxonomy may change in the future.
- Phylum Chordata – all chordates
- subphylum Cephalochordata – lancelets
- subphylum Tunicata (Urochordata) – tunicates
- clade Craniata – all chordates with a bony or cartilaginous skull
- subphylum Vertebrata – all chordates with a vertebral column
- Class Myxini – hagfish (do not have a vertebral column, but sometimes still grouped with Vertebrata as vertebrae were possibly lost during evolution)
- Class Petromyzontida – lampreys
- infraphylum Gnathostomata – jawed vertebrates
- Class Chondrichthyes – sharks, rays, skates, chimaeras
- superclass Osteichthyes – bony fish
- Class Actinopterygii – ray-finned fish (most bony fish)
- Class Sarcopterygii – lobe-finned fish (lungfish, coelacanths)
- superclass Tetrapoda – tetrapods
- Class Amphibia – amphibians
- Class Reptilia – reptiles (including birds, although they are still sometimes given their own Class, Aves)
- Class Mammalia – mammals
- Class Myxini – hagfish (do not have a vertebral column, but sometimes still grouped with Vertebrata as vertebrae were possibly lost during evolution)
- subphylum Vertebrata – all chordates with a vertebral column
Hemichordates
Hemichordates are small marine species that mostly live in either excavated burrows or secreted tubes on the seafloor. There are two classes, Class Enteropneusta and Class Pterobranchia. Class Enteropneusta, commonly known as the acorn worms, are best known to science. Like chordates, they have a dorsal, hollow nerve cord and pharyngeal gill slits. Molecular analyses place the hemichordates as a sister taxon to the echinoderms.
Echinoderms
Echinoderms are a phylum of exclusively marine animals. Most are benthic, meaning they inhabit the seafloor. They are unusual triploblastic animals that usually have pentaradial symmetry as adults. However, as larvae, they exhibit bilateral symmetry and are thus classified as bilaterians. Molecular data place them as close relatives to chordates, within the deuterostome clade.
Echinoderms have an endoskeleton composed of calcareous plates. They also have a water vascular system, which consists of a series of canals that accomplishes the tasks of circulation, feeding, and motility. Water enters through a madreporite, usually on the dorsal aspect of the animal, and enters stone canal, which feeds into a central a ring canal. From there, it is distributed into the radial canals. In echinoderms with arms, the radial canals run along the arms. Many echinoderms have tube feet which allow the organism to move and, in the case of sea stars, pry open bivalves. Sea stars can also evert their stomachs and use them to digest bivalves outside of the organism. Echinoderms usually reproduce sexually and are dioecious, although some species are hermaphroditic.
There are about 7,000 species of echinoderms in (currently) five classes.
- Class Crinoidea – sea lilies, feather stars
- Class Asteroidea – sea stars/starfish
- Class Ophiuroidea – brittle stars
- Class Echinoidea – sea urchins, sand dollars
- Class Holothuroidea – sea cucumbers
[1] Simion et al. (2017). A large and consistent phylogenomic dataset supports sponges as the sister group to all other animals. Current Biology 27, 958–967
[2] Nüchter et al. (2006). Nanosecond-scale kinetics of nematocyst discharge. Current Biology 16, R316-R318.
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