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| Crustaceans | ||||||
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Hyalella azteca | ||||||
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Remipedia Cephalocarida Branchiopoda Ostracoda Maxillopoda Malacostraca |
The crustaceans (Crustacea) are a large group of arthropods (55,000 species), usually treated as a subphylum. They include various familiar animals, such as lobsters, crabs, shrimp and barnacles. They are variously found in marine and freshwater, with a few terrestrial members (such as woodlice).
Crustaceans have three distinct body parts: head, thorax, and abdomen. They have two pairs of antennae on the head, compound eyedragonfly A compound eye is a visual organ found in certain arthropods (some insects and crustaceans). The compound eye consists of between 12 and 1,000 ommatidia, little dark/bright sensors. The image perceived by the arthropod is "recalculated" from thes, three pairs of mouthparts and a telsonFinlike structure at the back of many crustaceans Function: Escape reaction: Krill can evade predators by very fast backward swimming ( lobstering), flipping its telson. They reach speeds of over 60 cm per second. The trigger time to optical stimulus is i. Smaller crustaceans respire through their body surface by diffusionThis article is about the physical mechanism of diffusion. For alternative meanings, see diffusion (disambiguation). Diffusion is the spontaneous spreading of something such as particles, heat, or momentum. The phenomenon is readily observed when a drop o and larger crustaceans respire with gillGill In aquatic organisms In aquatic organisms, gills are a respiratory organ for the extraction of oxygen from water and for the excretion of carbon dioxide. Many small aquatic animals absorb oxygen through the surface of their bodies in general, but mors. Crustaceans typically have a thick carapace on the dorsal side of their body. Their appendages are typically biramous, including the second pair of antennae (but not the first).
Most crustaceans have separate sexes and are distinguished by appendages on the abdomen called swimmerets. The first (and sometimes the second) pair of swimmerets are larger on the male than on the female. Terrestrial crabs mate seasonally and return to the sea to breed. Female crabs' eggs are retained by the females until they hatch. The eggs hatch into free-swimming larvaeLarvae are the plural of larva, juvenile form of animals with indirect development. See larva In Roman mythology, the Larvae were the spectres or spirits of the dead; they were the malignant version of the Lares. Some Roman writers describe Lemures as the.
The formal classification of crustaceans varies somewhat. In general, because of the large number of species, taxonomists have made extensive use of subordinate taxonomic categories ( suborderIn biology, a suborder is a subdivision of an order. Scientific classification.s, superfamiliesIn biology, a superfamily is a taxonomic grade intermediate between suborder and family. Plant superfamilies end with the suffix acea animal superfamilies end with oidea (although in older books the suffix acea is still used). Superfamily can also refer t and so forth), and the status of different groupings is frequently controversial; this can make taxonomic references hard to follow. Evolutionary relationships between the different groups are not entirely clear, making the exact definition of larger groups difficult.
Some authorities have treated the entire group of crustaceans as a class, in which case the classes escribed in the taxonomic table are treated as subclasses. Other authors omit some or all of the classes listed here, in which case some of the groups given here as subclasses are promoted to full class rank. This listings given at the end of this article and in the summary table at the right are those recommended by ITIS, and as such probably represent a consensus of modern opinion. However good practice in describing crustaceans must clearly be to include descriptions at several taxonomic levels, to ensure that readers can link the information to others' schemes.
Less formally, we can state that the most important groups of crustaceans are barnacles (infraorder Cirripedia), branchiopods, copepods and Malacostraca ( crabs, lobsters, shrimps and krill). There are around 1,220 barnacle species, 1,000 branchiopods, 13,000 copepods, and 30,000 Malacostraca.
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