Supergroup Opisthokonta – Protostomes – Background Reading

This lab covers the protostomes, which include the platyhelminths (flatworms), rotifers, molluscs, annelids, tardigrades, nematodes (roundworms), and arthropods. Protostomes are bilaterally symmetrical, triploblastic animals. They differ from deuterostomes in several important ways. Cleavage is spiral, rather than radial. Cleavage is also determinate, meaning that each cell has a specific fate from the earliest stages of development. The blastopore develops into the mouth (or into the single gut opening if the organism has an incomplete digestive system). Mesoderm develops from a single blastomere located near the blastopore. Protostomes are far more numerous than deuterostomes, mainly due to the inclusion of the arthropods (although mollusc species alone outnumber deuterostome species).


Lophotrochozoans are one of the two major clades of deuterostomes (Ecdysozoa) is the other. The word lophotrochozoa is a contraction of two words – lophophore and trochophore. The lophophore is a feeding structure that generally consists of ciliated tentacles. The trochophore is a particular type of larva that is free-swimming due to a band of cilia that girdles it. Some lophotrochozoans have one of these structures; others have neither and are placed within the group due to molecular homology. Lophotrochozoans include Phylum Platyhelminthes, Phylum Rotifera, Phylum Mollusca, Phylum Annelida, and a number of other phyla.


Flatworms, or platyhelminths, belong to Phylum Platyhelminthes. They are bilaterally symmetrical, triploblastic, acoelomate animals. Their body plan is relatively simple. They are cephalized, and usually have a single brain located in the head region. The brain communicates with the rest of the body via two ventral nerve cords that run the length of the body. 

Flatworms generally have a two-way digestive system, with a single opening that functions as both mouth and anus, although some species have a complete digestive system with an anus (or sometimes, more than one anus). Tapeworms have lost their digestive system altogether and rely on absorption to obtain nutrients. Flatworms lack specialized respiratory and circulatory systems, relying on diffusion for transport of oxygen, carbon dioxide, and nutrients.

Platyhelminths exhibit several modes of reproduction. Some species of planarian have never been observed to reproduce sexually. Other flatworms are dioecious, with separate male and female individuals. However, most flatworms are hermaphrodites, producing both eggs and sperm.

There are about 29,000 flatworm species. Some are free-living, inhabiting marine, freshwater, or terrestrial ecosystems. Others are parasitic, and have life cycles involving one, two, or three hosts. Many are significant human parasites, including tapeworms, liver flukes, and blood flukes. 


Rotifers are bilaterally symmetrical, triploblastic animals. They are tiny – most cannot be seen without a microscope. Despite their size (no more than 2mm), they have a complex anatomy. Their nervous system includes a brain and one to five eyes. Rotifers have a complete digestive system with a mouth, stomach, intestine, and anus. The mouth is surrounded by a corona – a crown of cilia that beat to produce a current that directs water and food particles into the mouth. 

Rotifers may be sessile or free-living. They are mostly freshwater organisms, but some are marine, or live in wet terrestrial environments. Rotifers that reproduce sexually are dioecious; males and females are separate individuals. However, the bdelloid rotifers are famous for reproducing solely by asexual (mitotic) parthenogenesis. All bdelloid rotifers are female. A third type of rotifers can switch between sexual reproduction and parthenogenesis. There are about 2,200 species of rotifers, although their classification is in flux.


Phylum Mollusca is a large clade of bilaterian, triploblastic animals that inhabit marine, freshwater, and terrestrial ecosystems. Molluscs are characterized by three main body parts: a foot, a visceral mass, and a mantle, which may secrete a shell. Molluscs have a complex nervous system with a brain and two nerve cords (although bivalves are an exception, with three ganglia and three nerve cords). Most have eyes, and the cephalopods have extremely well-developed, camera-like eyes. The digestive system is complete. The mouth is often equipped with a rasp-like tongue, called the radula. They have an open circulatory system (except for cephalopods).

Reproduction varies within molluscs. Gastropods and bivalves may be either dioecious or hermaphroditic. Hermaphrodites may self- or cross-fertilize. Chitons and cephalopods are dioecious, and sexual selection and sexual dimorphism is present in some species. Marine molluscs often have a trochophore larval phase before taking on the characteristics of the adult.

There are around 85,000 described mollusc species. There are five well-characterized extant classes, plus two minor class that may be polyphyletic (and are not listed below).

Mollusc groups

  • Class Polyplacophora – chitons
  • Class Bivalvia – mussels, oysters, clams, scallops, etc.
  • Class Gastropoda – snails, slugs, nudibranchs, limpets, abalone
  • Class Cephalopoda – octopus, cuttlefish, squid, nautilus
  • Class Scaphopoda – tusk shells


Annelids are bilaterally symmetrical, triploblastic worms with a true coelom. Annelids are also often referred to as the segmented worms. Most are indeed segmented; however, a sizeable minority of species show no segmentation. In fact, two groups of unsegmented worms (Echiura and Sipuncula) that were considered to be separate phyla until recently, have now been reclassified as annelids based on molecular phylogeny.[1]  In species that are segmented, most segments are almost exactly the same, and contain a number of specialized organs. Septa separate the body segments. Most annelids have a complete gut. The circulatory system is usually closed. The nervous system usually consists of a brain and two nerve cords that run the length of the body, although many species have fused the two nerve cords into one. Many marine species have eyes, either compound or camera-like.

Reproduction is generally sexual with a variety of life cycles. Earthworms and leeches are hermaphrodites, but many marine annelids are dioecious. Some species are sequential hermaphrodites, meaning they switch sexes at some point during their life. 

There are about 22,000 described species of annelids. Most textbooks still describe polychaetes, oligochaetes, and leeches as separate groups. The polychaetes, which are characterized by protrusions extending from each segment (parapodia), are not a monophyletic group and need to be reclassified. Oligochaetes (including earthworms) and leeches belong to the same class, Clitellata, which also includes the echiurans (spoon worms).


Ecdysozoans are animals that undergo ecdysis. Ecdysis is a specific type of molting and replacing of the external protective cuticle, or exoskeleton. Organisms that undergo ecdysis include tardigrades, nematodes, arthropods, onycophorans, and a few other minor phyla.


Tardigrades, or water bears, are microscopic animals that are famous for the ability to withstand extreme environmental conditions. They are bilaterian, triploblastic, and coelomate. They are distributed worldwide and inhabit freshwater, marine, and terrestrial environments. They are often found on moss and lichens. There are more than 1,000 described species.

Tardigrades can enter a dormant state called a tun. While in this state, their metabolism is almost completely halted. While in the tun state, tardigrades have been observed to survive freezing, complete dehydration, pressures ranging from a vacuum to 1,200 atmospheres, radiation a thousand times greater than a human can survive (5,000+ Gy), temperatures up to 150°C, and the conditions of outer space.


Nematodes are bilaterian, triploblastic, pseudocoelomate worms. They range from microscopically tiny to over 8 meters long (Placentonema gigantissima, a parasite of sperm whales). Nematodes have an outer covering called the cuticle, which protects them from dehydration. The cuticle is molted as the animal grows. The nervous system includes a brain that is ring-shaped and surrounds the pharynx, and a ventral nerve cord. Nematodes have a complete gut, with a mouth at the anterior end and an anus near the posterior end, but without a stomach. The muscular pharynx connects directly to the intestine.

Nematodes are often dioecious, but a few species, including the best-studied (Caenorhabditis elegans) are androdioecious, meaning there are hermaphrodites and males. Some nematodes reproduce via parthenogenesis. Nematodes inhabit almost every ecosystem on earth. Many are parasitic, and a few species are common and significant human parasites. There are about 25,000 described species.


Arthropods are the most numerous of animals, and account for over 80% of described animal species. They are distributed worldwide and inhabit marine, freshwater, terrestrial, and aerial ecosystems. Although each group has unique anatomical features, all arthropods share several distinguishing characteristics. All arthropods have an exoskeleton made of chitin, which is shed via ecdysis multiple times during the arthropod’s lifespan. Arthropods display segmentation of the body. Body regions of arthropods are called tagma. For instance, insects have three tagmata – the head, thorax, and abdomen. Jointed appendages are another key characteristic of all arthropods.

Arthropods have a central cavity that contains the blood, called the hemocoel, and the open circulatory system is regulated by a tubular or single-chambered heart. 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.

Most arthropods reproduce sexually, although some can reproduce by parthenogenesis. A few arthropods are hermaphrodites, including some barnacles. However, most are dioecious, with distinct male and female sexes. Some display sexual dimorphism and sexual selection. Most arthropods lay eggs, but some are viviparous, giving birth to live young. Some species of scorpions and spiders exhibit parental care.

Major Groups of Arthropods:


The earliest trilobites appear in the fossil record at the beginning of the Cambrian Period, around 521 million years ago. In fact, trilobites are so widespread and common that they are used by stratigraphers to data Paleozoic strata.

Trilobites had a segmented body, with three tagmata – the head, thorax, and pygidium (tail piece). The thorax has many segments. Trilobite fossils are generally only of the exoskeleton, which was chitinous and reinforced with calcite. Trilobites had jointed appendages and a single pair of antennae. 

By the time trilobites appear in the fossil record, however, they were already very diverse and widely distributed, suggesting that they evolved prior to the Cambrian Period. The classification of trilobites is disputed and may be impossible to resolve since no molecular data are available; the tree shown is one hypothesis, other groups place trilobites as the closest relative to chelicerates.


Crustaceans are a diverse group of arthropods. Most are marine, but there are also freshwater and terrestrial crustaceans. This group includes Class Malacostraca (lobsters, crabs, krill, shrimp, crayfish, amphiopods, etc.), Class Ostracoda (seed shrimp), Class Branchiopoda (fairy shrimp, clam shrimp, etc.), Class Maxillopoda(barnacles, copepods, etc.), and others. 

Crustaceans have two tagmata, a cephalothorax and an abdomen. The cephalothorax is covered by a plate called the carapace. Crustaceans have a chitinous exoskeleton that is shed by molting whenever the animal increases in size. 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 heart, which is in the dorsal aspect of the thorax.


Subphylum Myriapoda includes 13,000 described species and is comprised of arthropods with numerous legs. Despite the names centipede and millipede, these arthropods do not have 100 or 1000 feet. The number of legs may vary from 10 to 750. All myriapods are terrestrial animals and prefer a humid environment.

Myriapods are typically found in moist soils, decaying biological material, and leaf litter. Centipedes bear one pair of legs per segment, mandibles as mouthparts, and are somewhat dorsoventrally flattened. The legs in the first segment are modified to form forcipules (poison claws) that deliver poison to prey like spiders and cockroaches, as these animals are all predatory. Millipedes bear two pairs of legs per diplosegment, a feature that results from embryonic fusion of adjacent pairs of body segments, are usually rounder in cross-section, and are herbivores or detritivores. Millipedes generally have more legs as compared to centipedes. Myriapods have eyes called stemmata, which are thought to have evolved from compound eyes.


The name Hexapoda denotes the presence of six legs (three pairs). Hexapods are characterized by three tagmata — head, thorax, and abdomen. The thorax bears the wings and legs. Amongst the hexapods (and arthropods, and animals), insects are by far the largest class in terms of numbers, species diversity, and biomass. There are over 1 million described species, which is certainly a gross underestimate of total insect diversity. Insects are extremely successful for several reasons. First, they were among the first animals to colonize the land. Second, they have evolved true flight, allowing them to occupy a variety of habitats, escape predators, and search for mates. Third, they coevolved with the seed plants, particularly the angiosperms. 

Insects typically have one pair of sensory antennae, mandibles as mouthparts, a pair of compound eyes, and some ocelli (simple eyes) along with numerous sensory hairs. The thorax bears three pairs of legs and two pairs of wings in most insect groups, although some have lost one or both pairs of wings. The nervous system of insects is complex, including a number of ganglia and a ventral, double nerve cord. Sense organs are complex and acute. In addition to ocelli and compound eyes, some insects are quite sensitive to sounds, and their chemoreceptive abilities can be astounding. Insects are dioecious and fertilization is internal in most. 

Insects undergo metamorphosis. In holometabolous insect species, newly hatched young are completely different in appearance from adults. These larval forms usually have a different morphology and ecological niche than adults. When larval growth is completed, the larva stops feeding and builds a case or cocoon around itself. While encased, the larva undergoes a complete transformation, and a fully-formed adult emerges. Hemimetabolous species undergo a more gradual process, in which the newly hatched young are more similar to the adult but are small in size, lack wings, are sexually immature, and may differ in other, relatively minor ways as well. The young in these insects are called nymphs.

[1] Struck et al. (2011) Phylogenomic analyses unravel annelid evolution. Nature 471:95.