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Photo a shows a crab on land, and photo b shows a bright red shrimp in the water.
The (a) crab and (b) shrimp krill are both crustaceans. (credit a: modification of work by William Warby; credit b: modification of work by Jon Sullivan)

Crustaceans possess two pairs of antennae, mandibles as mouthparts, and biramous    (“two branched”) appendages, which means that their legs are formed in two parts, as distinct from the uniramous    (“one branched”) myriapods and hexapods ( [link] ).

 Illustration A shows the biramous, or two-branched leg of a crayfish. Illustration B shows the uniramous, or one-branched leg of an insect.
Arthropods may have (a) biramous (two-branched) appendages or (b) uniramous (one-branched) appendages. (credit b: modification of work by Nicholas W. Beeson)

Unlike that of the Hexapoda, the head and thorax of most crustaceans is fused to form a cephalothorax    ( [link] ), which is covered by a plate called the carapace, thus producing a body structure of two tagma. 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 dorsally located heart. Hemocyanin and hemoglobin are the respiratory pigments present in these animals.

An illustration of a midsagittal cross section of a crayfish shows the carapace around the cephalothorax, and the heart in the dorsal thorax area.
The crayfish is an example of a crustacean. It has a carapace around the cephalothorax and the heart in the dorsal thorax area. (credit: Jane Whitney)

Most crustaceans are dioecious, which means that the sexes are separate. Some species like barnacles may be hermaphrodites . Serial hermaphroditism, where the gonad can switch from producing sperm to ova, may also be seen in some species. Fertilized eggs may be held within the female of the species or may be released in the water. Terrestrial crustaceans seek out damp spaces in their habitats to lay eggs.

Larval stages— nauplius    and zoea    —are seen in the early development of crustaceans. A cypris    larva is also seen in the early development of barnacles ( [link] ).

Micrograph a shows a shrimp nauplius larva, which has a teardrop-shaped body with tentacles and long, frilly arms at the wide end. Micrograph b shows a barnacle cypris larva, which is similar in shape to a clam. Micrograph c shows green crab zoea larva, which resembles a shrimp.
All crustaceans go through different larval stages. Shown are (a) the nauplius larval stage of a tadpole shrimp, (b) the cypris larval stage of a barnacle, and (c) the zoea larval stage of a green crab. (credit a: modification of work by USGS; credit b: modification of work by Mª. C. Mingorance Rodríguez; credit c: modification of work by B. Kimmel based on original work by Ernst Haeckel)

Crustaceans possess a tripartite brain and two compound eyes. Most crustaceans are carnivorous, but herbivorous and detritivorous species are also known. Crustaceans may also be cannibalistic when extremely high populations of these organisms are present.

Subphylum chelicerata

This subphylum includes animals such as spiders, scorpions, horseshoe crabs, and sea spiders. This subphylum is predominantly terrestrial, although some marine species also exist. An estimated 77,000 species are included in subphylum Chelicerata. Chelicerates are found in almost all habitats.

The body of chelicerates may be divided into two parts: prosoma and opisthosoma, which are basically the equivalents of cephalothorax (usually smaller) and abdomen (usually larger). A “head” tagmum is not usually discernible. The phylum derives its name from the first pair of appendages: the chelicerae ( [link] ), which are specialized, claw-like or fang-like mouthparts. These animals do not possess antennae. The second pair of appendages is known as pedipalps . In some species, like sea spiders, an additional pair of appendages, called ovigers , is present between the chelicerae and pedipalps.

The photo shows a black, shiny scorpion with very large chelicerae, or pincers.
The chelicerae (first set of appendages) are well developed in the scorpion. (credit: Kevin Walsh)

Chelicerae are mostly used for feeding, but in spiders, these are often modified into fangs that inject venom into their prey before feeding ( [link] ). Members of this subphylum have an open circulatory system with a heart that pumps blood into the hemocoel. Aquatic species have gills, whereas terrestrial species have either trachea or book lungs for gaseous exchange.

The photo shows a spider with a thick, hairy body and eight long legs.
The trapdoor spider, like all spiders, is a member of the subphylum Chelicerata. (credit: Marshal Hedin)

Most chelicerates ingest food using a preoral cavity formed by the chelicerae and pedipalps. Some chelicerates may secrete digestive enzymes to pre-digest food before ingesting it. Parasitic chelicerates like ticks and mites have evolved blood-sucking apparatuses.

The nervous system in chelicerates consists of a brain and two ventral nerve cords. These animals use external fertilization as well as 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.

Visit this site to click through a lesson on arthropods, including interactive habitat maps, and more.

Section summary

Nematodes are pseudocoelomate animals akin to flatworms, yet display more advanced neuronal development, a complete digestive system, and a body cavity. This phylum includes free-living as well as parasitic organisms like Caenorhabditis elegans and Ascaris spp., respectively. They include dioeceous as well as hermaphroditic species. Nematodes also possess an excretory system that is not quite well developed. Embryonic development is external and proceeds via three larval stages. A peculiar feature of nematodes is the secretion of a collagenous/chitinous cuticle outside the body.

Arthropods represent the most successful phylum of animal on Earth, in terms of the number of species as well as the number of individuals. These animals are characterized by a segmented body as well as the presence of jointed appendages. In the basic body plan, a pair of appendages is present per body segment. Within the phylum, traditional classification is based on mouthparts, number of appendages, and modifications of appendages present. Arthropods bear a chitinous exoskeleton. Gills, trachea, and book lungs facilitate respiration. Sexual dimorphism is seen in this phylum, and embryonic development includes multiple larval stages.

Art connections

[link] Which of the following statements about insects is false?

  1. Insects have both dorsal and ventral blood vessels.
  2. Insects have spiracles, openings that allow air to enter.
  3. The trachea is part of the digestive system.
  4. Insects have a developed digestive system with a mouth, crop, and intestine.

[link] C

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Questions & Answers

What is cytoplasm
Nitta Reply
Parasitic adaptation of tapeworm
Angela Reply
Describe osmoregulator or osmoconformers and how these tools allow animals to adapt to different environment.
Alick Reply
describe the secondary function of a leaf to a plant.
Twizera Reply
hi I'm asking a question about HIV infection, can HIV infection transmitted from Mother to unbron child? please help me I So confused.
Khushboy Reply
no it can't
yes it can but their is an injection that can be used to prevent it.
Ahmad how no?
Hi im new
no, unless when giving birth
it is can be transmitted but there is an injection that the mother is injected to prevent the disease
I agree with nia
what is the difference between primary and secondary active transport in detail? I didn't understand the steps in the textbook specifically
Fathima Reply
you are a doctor?
what is the meaning of connective tissue?
Mohammed Reply
what are the characteristics of living things
Owolo Reply
what's the meaning of connective tissue?
Reproduction, adaptation, interaction, movement, growth, respiration, made of cells, responsive to environment (homeostasis), metabolic action (consumption of food converted into energy)
Movement, reproduction, nutrition, irritability, growth, excretion, respiration, death, adaptation, competition
state two most important factors that favour exponential growth of population of a gazelle in a pack
Eliza Reply
what are the two types of electron microscope
Sharlom Reply
light microscope and early microscope
Enzymes are biological catalyst which alter any reaction and protein in nature
Nkoue Reply
Your welcome sir
guyz you enjoying
What is translation and transcription
Transcription is making RNA from DNA. Translation is going from RNA to proteins.
full meaning of RNA and DNA
Nkoue what homeostits means?
Mohammed khalfan, In biology, homeostasis is the state of steady internal, physical, and chemical conditions maintained by living systems.[1] This dynamic state of equilibrium is the condition of optimal functioning for the organism and includes many variables, such as body temperature and fluid bal
what is the definition of enzymes
Royd Reply
enzymes are biological catalyst that speed up chemical reaction.
What are enzymes?
Enzymes are made of proteins and lower the energy of activation. In other words, they bring things together which helps to lower the amount of energy for a reaction to go forward.
they are catalyses that speeds up chemical reaction.... e.g they break down the food we consume.
These are catalyst that speeds up the chemical reaction.
What is connective tissue?
what homeostits means ?
what is a spirogyra
Talabi Reply
Spirogyra is a filamentous chlorophyte green algae of the order Zygnematales. It is named for the helical or spiral arrangement of the chloroplasts. That is characteristic of the genus. It is commonly found in freshwater habitats. And there are more than 400 species of Spirogyra in the world.
what is the mean of biology
Bello Reply
what is cell
A cell is the smallest living unit.
Hi I'm new in this group can someone please help with the list features shared by plants and charopytes that are not shared with most other eukaryotes
iz a basic units of a living thing?
what is eutrophication
Chinaza Reply
hi.. I'm asking a question about HIV infection.... Can HIV infection transmitted from Mother to unbron child?
Eutrophication is an enrichment of water by nutrient salts that causes structural changes to the ecosystem such as: increased production of algae and aquatic plants, depletion of fish species, general deterioration of water quality and other effects that reduce and preclude use.

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