Animals: Invertebrates

Learning Objectives

  1. Place animals on a phylogenetic tree
  2. Identify and describe key adaptations of all animals except sponges
  3. Identify and use key features to differentiate between invertebrate groups, including poriferans, cnidarians, protostomes (lophotrochozoans and ecdysozoans), and the invertebrate deuterostomes (sea stars!)
  4. Organize the appearance and/or flourishing of major invertebrate groups in chronological order in geologic time

What makes an animal, an animal?

All animals are descended from a common ancestral protist. Although estimates are inexact, it is thought that multicellular animals first emerged around 800-900 million years ago, but it wasn’t until the Cambrian Explosion (around 500-540 million years ago) that animal life began to greatly diversify. Today, although only 1.4 million species have been identified, there are somewhere between 8-5o million animal species estimated to be alive today.

Overarching phylogenetic tree of animals and their ancestor. Note that this is one hypothesis for the evolution of animals based on the criteria shown in blue (credit: Emily Weigel)

With a few exceptions, all animals have the following in common:

  1. Multicellular body plans with cells that lack cell walls
  2. Heterotrophic means of acquiring nutrients
  3. Movement at some point in their life cycle
  4. Except for sponges, neurons and muscle cells that can transmit signals and change the body shape

Now, what you might generally picture in your head as an animal may be your dog, a bird, or a fish, or another species with a backbone. However, concentrating on vertebrates gives us a rather biased and limited view of biodiversity because it ignores nearly 97 percent of all animals, namely the invertebrates. Invertebrate animals are those without a cranium and defined vertebral column or spine. In addition to lacking a spine, most invertebrates also lack an endoskeleton.


In the following sections, we will review the key features used to differentiate invertebrate groups.

Study tip: As you read this section, use the above phylogenetic tree to organize the groups.

Porifera (Sponges)

The information below was adapted from OpenStax Biology 28.1

The simplest of all the invertebrates, Porifera (sponges) do not display tissue-level organization, although they do have specialized cells that perform specific functions. Sponge larvae are able to swim; however, adults are non-motile and spend their life attached to a substratum. Since water is vital to sponges for excretion, feeding, and gas exchange, their body structure facilitates the movement of water through the sponge.

Watch this video to see the movement of water through the sponge body.

Cnidaria (Sea Anemones, Corals, Jellyfish, and Box Jellies)

The information below was adapted from OpenStax Biology 28.2

The phylum Cnidaria contains about 10,000 described species divided into four classes: Anthozoa, Scyphozoa, Cubozoa, and Hydrozoa. The anthozoans, the sea anemones and corals, are all sessile species, whereas the scyphozoans (jellyfish) and cubozoans (box jellies) are swimming forms. The hydrozoans contain sessile forms and swimming colonial forms. The phylum Cnidaria includes animals that show radial or biradial symmetry and are diploblastic, that is, they develop from two embryonic layers. Nearly all (about 99 percent) cnidarians are marine species.

Animals in this phylum display two distinct morphological body plans: polyp or medusa. An example of the polyp form is Hydra spp.; perhaps the most well-known medusoid animals are the jellies (jellyfish). Polyp forms are sessile as adults, with a single opening to the digestive system (the mouth) facing up with tentacles surrounding it. Medusa forms are motile, with the mouth and tentacles hanging down from an umbrella-shaped bell. Still, some cnidarians are polymorphic, that is, they have two body plans during their life cycle (e.g., Obelia).

Cnidarians have two distinct body plans, the medusa (a) and the polyp (b). All cnidarians have two membrane layers, with a jelly-like mesoglea between them. Credit: Open Stax

Cnidarians contain specialized cells known as cnidocytes (‘stinging cells’) containing organelles called nematocysts (stingers). These cells are present around the mouth and tentacles, and serve to immobilize prey with toxins contained within the cells. The cnidarians then perform extracellular digestion in which the food is taken into the gastrovascular cavity, enzymes are secreted into the cavity, and the cells lining the cavity absorb nutrients. The gastrovascular cavity has only one opening that serves as both a mouth and an anus, which is termed an incomplete digestive system. There is no explicit excretory system nor circulatory system, thus wastes and gases must simply diffuse from the cells into the water outside the animal or in the gastrovascular cavity. All cnidarians show the presence of two membrane layers in the body that are derived from the endoderm and ectoderm of the embryo, and have differentiated cell types.

Protostomes: Lophotrochozoa and Ecdysozoa

The information below was adapted from OpenStax Biology 28.3

Study tip: As you read this section, keep the learning objectives in mind. We introduce details relevant for future modules as we talk about these organisms, but narrow your focus to the objectives above.

Protostomes are animal in which the blastopore, or the point of involution of the ectoderm or outer germ layer, becomes the mouth opening to the future gut. This is called protostomy or ‘first mouth.’ In protostomy, solid groups of cells split from the endoderm or inner germ layer to form a central mesodermal layer of cells. This layer multiplies into a band and then splits internally to form the coelom, or body cavity.

Lophotrochozoa (Flatworms, Rotifers, Worms, and Molluscs)

The lophotrochozoans have three cell layers (triploblastic), as they possess an embryonic mesoderm sandwiched between the two cell layers (ectoderm and endoderm) found in the diploblastic cnidarians. These phyla are also bilaterally symmetrical, meaning that a longitudinal section will divide them into right and left sides that are symmetrical. It also means the beginning of cephalization, the evolution of a concentration of nervous tissues and sensory organs in the head of the organism, which is where it first encounters its environment.

Phylum Platyhelminthes (Flatworms)

Most of the flatworms are classified in the superphylum Lophotrochozoa. The flatworms are acoelomates (no coelom), so their bodies are solid between the outer surface and the cavity of the digestive system. There is neither a circulatory nor respiratory system, with gas and nutrient exchange dependent on diffusion and cell-cell junctions. This necessarily limits the thickness of the body in these organisms, constraining them to be flat.

Most flatworm species are monoecious, and fertilization is typically internal. Asexual reproduction is common in some groups.

A marine flatworm in East Timor. Credit: Nick Hobgood.

The flatworms also include many free-living and parasitic forms, including important parasites of humans.

Phylum Rotifera

The rotifers are a microscopic (about 100 µm to 30 mm) group of mostly aquatic organisms that get their name from the corona, a rotating, wheel-like structure that is covered with cilia at their head. Rotifers obtain their food by the current created by the movement of the corona. The rotifers are filter feeders that will eat dead material, algae, and other microscopic living organisms, and are therefore very important components of aquatic food webs.

The body form of rotifers consists of a head (which contains the corona), a trunk (which contains the organs), and the foot. Rotifers are typically free-swimming and truly planktonic organisms, but the toes or extensions of the foot can secrete a sticky material forming a holdfast to help them adhere to surfaces. The head contains sensory organs in the form of a bi-lobed brain and small eyespots near the corona.

Shown are examples from two of the three classes of rotifer. (a) Species from the class Bdelloidea are characterized by a large corona, shown separately from the whole animals in the center of this scanning electron micrograph. (b) Polyarthra, from the class Monogononta, has a smaller corona than Bdelloid rotifers, and a single gonad, which give the class its name. (credit a: modification of work by Diego Fontaneto; credit b: modification of work by U.S. EPA; scale-bar data from Cory Zanker)

Rotifers are pseudocoelomates (partial coelom) commonly found in freshwater and some salt water environments throughout the world. Rotifers are dioecious organisms (having either male or female genitalia) and exhibit sexual dimorphism (males and females have different forms). Many species are parthenogenic and exhibit haplodiploidy, a method of sex determination in which a fertilized egg develops into a female and an unfertilized egg develops into a male. In many species, males are short-lived and smaller with no digestive system and a single testis. Females can produce eggs that are capable of dormancy for protection during harsh environmental conditions.

Study tip: Following along with your phylogenetic tree? Notice here that, while rotifers are most closely related to nematodes, due to a process called molting (shedding skin), nematodes are not categorized  (named) under Lophotrochozoa, but rather Ecdysozoa, along with Arthropods below. We will cover the remainder of the Lophotrochozoa before introducing the Ecdysozoa.

Phylum Mollusca

Phylum Mollusca is predominantly a marine group of animals; however, they are known to inhabit freshwater as well as terrestrial habitats. It is estimated that 23 percent of all known marine species are mollusks, making them the second most diverse phylum of animals. You may know them as clams, oysters, mussels, scallops, snails, slugs, conchs, as well as octopi, squids, cuttlefish, and ammonites.

Mollusks display a wide range of morphologies in each class and subclass, but share a few key characteristics: a muscular foot used for anchorage, a visceral mass containing internal organs, and a mantle that may or may not secrete a shell of calcium carbonate.

(a) Snails and (b) slugs are both mollusks, but slugs lack a shell. (credit a: modification of work by Murray Stevenson; credit b: modification of work by Rosendahl)
There are many species and variations of mollusks; this illustration shows the anatomy of an aquatic gastropod mollusk. By Original by Al2, English captions and other edits by Jeff Dahl – Own work, CC BY-SA 4.0,

Mollusks are eucoelomate (a true coelom), but the coelomic cavity is restricted to a cavity around the heart in adult animals. These organisms possess a visceral mass containing their digestive, nervous, excretory, reproductive, and respiratory systems. Mollusk species that are exclusively aquatic have gills for respiration, whereas some terrestrial species have lungs for respiration. Shelled mollusks are specialized to secrete a chitinous and hard calcareous shell.

Sexual dimorphism is seen in this class of animals. Members of a species mate, and the female then lays the eggs in a secluded and protected niche. Females of some species show parental care, and some species hatch eggs which produce juvenile adults, skipping earlier life stages completely!

Phylum Annelida (Worms)

Annelida includes the segmented earthworms we typically mean when we say ‘worm’ colloquially, but polychaete worms and leeches belong to this group as well. These animals are found in marine, terrestrial, and freshwater habitats, but a presence of water or humidity is a critical factor for their survival, especially in terrestrial habitats.  Animals in this phylum show parasitic and commensal symbioses with other species in their habitat.

Annelids show protostomic development in embryonic stages and display bilateral symmetry. Key to this group, annelids have a segmented body plan wherein the internal and external morphological features are repeated in each body segment. This feature allows animals to become bigger by adding ‘compartments’ while making their movement more efficient. The overall body can be divided into head, body, and pygidium (or tail).

Although there are some exceptions, annelids generally possess many complex features:

  • A true coelom, derived from embryonic mesoderm and protostomy
  • A closed circulatory system of dorsal and ventral blood vessels that run parallel to the alimentary canal as well as capillaries that service individual tissues.
  • A well-developed nervous system including a nerve ring and nerve
  • A well-developed and complete digestive system, with a mouth, muscular pharynx, esophagus, crop, and gizzard (in oligochaetes and many others)

However, despite many complex features, annelids lack a well-developed respiratory system; instead, gas exchange occurs across the moist body surface. Excretion is facilitated by a pair of metanephridia (a type of primitive ‘kidney’ that consists of a convoluted tubule and an open, ciliated funnel) that is present in every segment towards the ventral side.

Annelids may be either monoecious (hermaphoroditic) with permanent gonads (as in earthworms and leeches) or dioecious (two distinct sexes) with temporary gonads that develop (as in polychaetes). However, cross-fertilization is preferred in hermaphroditic animals. These animals may also show simultaneous hermaphroditism and participate in simultaneous sperm exchange when they are aligned for copulation.

Ecdysozoa: Nematode Worms and Arthropods

The information below was adapted from OpenStax Biology 28.4

The superphylum Ecdysozoa contains an incredibly large number of species. The name derives from the word ecdysis, which refers to the shedding, or molting, of the exoskeleton. The phyla in this group have a hard cuticle that covers their bodies, which must be periodically shed and replaced for them to increase in size. The cuticle provides a tough, but flexible exoskeleton that protects these animals from water loss, predators and other aspects of the external environment. Ecdysozoa is so large because it contains two of the most diverse animal groups: phylum Nematoda (the roundworms) and Phylum Arthropoda (the arthropods).

Phylum Nematoda (Roundworms)

The Nematoda are triploblastic and possess an embryonic mesoderm that is sandwiched between the ectoderm and endoderm. They possess a pseudocoelom and are also bilaterally symmetrical. Furthermore, the phylum includes more than 28,000 species with an estimated 16,000 being parasitic in nature.  The free-living nematode, Caenorhabditis elegans has been extensively used as a model system in laboratories all over the world.

The overall morphology of these worms is cylindrical. The head is radially symmetrical. These animals show the presence of a complete digestive system with a distinct mouth and anus. This is in contrast with the cnidarians, where only one opening is present (an incomplete digestive system). The muscles of nematodes differ from those of most animals: they have a longitudinal layer only, which accounts for the whip-like motion of their movement.

Scanning electron micrograph shows (a) the soybean cyst nematode (Heterodera glycines) and a nematode egg. (b) A schematic representation shows the anatomy of a typical nematode. (credit a: modification of work by USDA ARS; scale-bar data from Matt Russell)

Phylum Arthropoda

The name Arthropoda means ‘jointed legs’ in Greek. It is the largest phylum in Animalia containing an estimated 85 percent of known species and many arthropods yet undocumented. Phylum Arthropoda includes animals that have been successful in colonizing terrestrial, aquatic, and aerial habitats. This phylum is further classified into five subphyla: Trilobitomorpha (trilobites, all extinct), Hexapoda (insects and relatives), Myriapoda (millipedes, centipedes, and relatives), Crustaceans (crabs, lobsters, crayfish, isopods, barnacles, and some zooplankton), and Chelicerata (horseshoe crabs, arachnids, scorpions, and daddy longlegs).

Study tip: We name 5 subphyla here of arthropods. Use the organisms of each subphyla as examples to help you remember the diversity contained in arthropods. The distinct ways each of these groups mate, garners nutrition, undergoes circulation, and respires will provide necessary background for future modules.

The principal characteristics of all the animals in this phylum are functional segmentation of the body and presence of jointed appendages. Arthropods also show the presence of an exoskeleton made principally of chitin, which is a waterproof, tough polysaccharide. Arthropods are eucoelomate, protostomic organisms, of which insects form the single largest class.

Respiratory systems vary depending on the group of arthropod: insects and myriapods use a series of tubes (tracheae) that branch through the body, open to the outside through openings called spiracles, and perform gas exchange directly between the cells and air in the tracheae, whereas aquatic crustaceans utilize gills, terrestrial chelicerates employ book lungs, and aquatic chelicerates use book gills.

Groups of arthropods also differ in the organs used for excretion, with crustaceans possessing green glands and insects using Malpighian tubules, which work in conjunction with the hindgut to reabsorb water while ridding the body of nitrogenous waste. Generally, a central cavity, called the hemocoel (or blood cavity), is present, and the open circulatory system is regulated by a tubular or single-chambered heart.

Subphylum Hexapoda

The name Hexapoda denotes the presence of six legs (three pairs) in these animals as differentiated from the number of pairs present in other arthropods. Hexapods are characterized by the presence of a head, thorax, and abdomen. Many of the common insects we encounter on a daily basis, including ants, cockroaches, butterflies, and flies, are examples of Hexapoda. This is also the largest class in terms of species diversity as well as biomass in terrestrial habitats. These organisms have note that insects have developed digestive, respiratory, circulatory, and nervous systems.

An example of a hexapod insect, the Yellow Jacket. Credit: D. Griebeling.

Subphylum Myriapoda

Myriapoda includes arthropods with numerous legs, varying from 10 to 750. This subphylum includes 13,000 species; the most commonly found examples are millipedes and centipedes. All myriapods are terrestrial animals and prefer a humid environment.

(a) The Scutigera coleoptrata centipede has up to 15 pairs of legs. (b) This North American millipede (Narceus americanus) bears many legs, although not a thousand, as its name might suggest. (credit a: modification of work by Bruce Marlin; credit b: modification of work by Cory Zanker)

Subphylum Crustacea

Crustaceans are the most dominant aquatic arthropods, since the total number of marine crustacean species stands at 67,000, but there are also freshwater and terrestrial crustacean species. Krill, shrimp, lobsters, crabs, and crayfish are examples of crustaceans. Terrestrial species like the wood lice (Armadillidium spp.), also called pill bugs, rolly pollies, potato bugs, or isopods, are also crustaceans, although the number of non-aquatic species in this subphylum is relatively low.

Crustaceans possess two pairs of antennae, mandibles as mouthparts, and head and thorax that is fused to form a cephalothorax. They also have biramous (‘two branched’) appendages, which means that their legs are formed in two parts, as distinct from the uniramous (‘one branched’) myriapods and hexapods.

Arthropods may have (a) biramous (two-branched) appendages or (b) uniramous (one-branched) appendages. (credit b: modification of work by Nicholas W. Beeson)

The exoskeletons of many species are also infused with calcium carbonate, which makes them even stronger than in other arthropods. Crustaceans have an open circulatory system where blood is pumped into the hemocoel by the dorsally located heart.

Species can come in hermaphroditic forms as well as with two distinct sexes, and most require some form of moisture or internal incubation by the mothers.

Subphylum Chelicerata

The Chelicerates include spiders, scorpions, horseshoe crabs, and sea spiders. This subphylum is predominantly terrestrial, although some marine species also exist. The phylum derives its name from the first pair of appendages: the chelicerae, which are specialized, claw-like or fang-like mouth parts. The body of chelicerates may be divided into two parts, with a relatively large abdomen and a comparatively smaller cephalothorax. These animals do not possess antennae.

Members of this subphylum have an open circulatory system with a heart that pumps blood into the hemocoel (a fluid cavity found in invertebrates). Aquatic species have gills, whereas terrestrial species have either trachea or book lungs for gaseous exchange. Most chelicerates ingest food using a preoral cavity, but some chelicerates may secrete digestive enzymes to pre-digest food before ingesting it, or make use of evolved blood-sucking apparatuses, as in mites and ticks.

These animals use external and internal fertilization strategies for reproduction, depending upon the species and its habitat. Parental care for the young ranges from absolutely none to relatively prolonged care.

 The Invertebrate Deuterostomes: Echinodermata

Different members of Echinodermata include the (a) sea star of class Asteroidea, (b) the brittle star of class Ophiuroidea, (c) the sea urchins of class Echinoidea, (d) the sea lilies belonging to class Crinoidea, and (e) sea cucumbers, representing class Holothuroidea. (credit a: modification of work by Adrian Pingstone; credit b: modification of work by Joshua Ganderson; credit c: modification of work by Samuel Chow; credit d: modification of work by Sarah Depper; credit e: modification of work by Ed Bierman)

The phyla Echinodermata and Chordata (the phylum in which humans are placed) both belong to the superphylum Deuterostomia. However, echinoderms are actually invertebrates, this group broke from the branch that would later develop a vertebral column in the chordate lineage.

Echinodermata are so named owing to their spiny skin (from the Greek “echinos” meaning “spiny” and “dermos” meaning “skin”), and are exclusively marine organisms. Sea stars, sea cucumbers, sea urchins, sand dollars, and brittle stars are all examples of echinoderms.

Adult echinoderms have a calcareous endoskeleton and exhibit pentaradial symmetry, although the early larval stages of all echinoderms have bilateral symmetry. Gonads are present in each arm. These animals possess a true coelom that is modified into a unique circulatory system called a water vascular system. By using hydrostatic pressure, the animal can either protrude or retract the tube feet to pump water to move and force open mollusk shells during feeding.

The nervous system in these animals is a relatively simple structure with a nerve ring at the center and five radial nerves extending outward along the arms. Echinoderms may also undergo external fertilization, asexual reproduction, and/regeneration of body parts lost in trauma.

 Key Events in Invertebrate History

Taking the timeline you’ve seen before, let’s zoom in to look at key events that have occurred. Note that the origins, or emergence, of a group may not align with when the group flourishes. Often, extinction events and the evolution of other organisms can open up new niches into which organisms will diversify.

Key events in the invertebrate timeline, as marked by blue text on the timeline. Note that most flourishing (diversifying) events are early and occur around the Cambrian, but are not limited to that time. (credit: Chrissy Spencer; adapted by Emily Weigel)