The phylum Arthropods contains almost a million species of free-living, parasitic or sedentary animals, and includes crustaceans, spiders and insects, as well as many smaller groups. An arthropod's exoskeleton, which is jointed to allow movement, is hardened and does not grow with the animal. As a result, it must be periodically shed - a process known as ecdysis - in order to allow growth. The arthropod body is usually divided into head, thorax and abdomen although in some groups there may be no a clear distinction between the 3 regions. The head and thorax may be fused, forming a cephalothorax, or the abdomen may be reduced in size. The head often carries feeding and sensory appendages. Each segment of he body usually has a pair of jointed appendages, which are modified for specific functions in different species.
Arthropods have a body cavity (coelom), but it is small and contains only gonads and excretory organs. The other internal cavities form a hemocoele (blood cavity). The circulatory system is open, with a dorsally situated heart. The digestive system consists of gut that runs from the mouth to the anus. The foregut and hindgut are lined with chitin and are shed at ecdysis. The nervous system has a dorsally positioned brain and a ventral nerve cord which has branches called ganglia in each segment. Tactile bristles are also a common feature. Eyes, which may be simple or compound are unique to arthropods and allow the formation of a highly-defined image.
Despite their name, horseshoe crabs (subclass Xiphosura) are not crabs at all, but primitive marine arthropods. They are the only living group of the class Merostomata, of which only 5 species remain. The body is divided into 3 main regions: the horseshoe-shaped prosoma (fused head and thorax), covered by a thick shell, or carapace, and ending in a long, pointed tail called a telson. On the ventral surface of the prosoma is a mouth with a pair of pincerlike feeding appendages called chelicerae, and 5 pairs of walking legs. The nervous system is well-developed, but the eyes are not; they can detect movement, although they cannot form an image.
PYCNOGONIDS. The class Pycnogonids contains about 600 species of carnivorous marine animals known as sea spiders. They feed on corals and sponges, and are found in all marine waters. The similarity of these animals to true spiders is only superficial. Pycnogonids have a segmented abdomen and legs which end in claws, whereas spiders do not have these chracteristics.
Chilopods and Diplopods
The animals in the class Chilopoda are commonly known as centipedes and those in class Diplopoda as millipedes. Centipedes have one pair of antennae on the head, a pair of mandibles and 2 pairs of amxillae, in contrasts to millipedes which have at least the fron pair of maxillae modified to form a lower lip. Centipedes have eyes that may be simple ocelli (a cluster of photoreceptors) or modified compound eyes. Centipedes are flattened and divided into a large number of segments, each of which, apart from the first, carries a pair of long, slender, walking legs.
Millipedes are vegetarian scavengers. Their body is also segmented, but is usually cylindrical. Millipedes and centipedes are found mainly in damp conditions such as rotting logs or in leaf litter, because they do not possess a waxy cuticle with which to reduce water loss. Both groups have separate sexes, and the female lays eggs which are fertilized by the male. Some species lay in the eggs in a "nest" where they are guarded by the female, but others, such as the centipede Lithobius, lay one egg at a time and then leave it.
Common house centipedeScutigera coleoptrata
Each segment of a centipede's body possesses a single pair of legs. Emanating from the head is a pair of sensorial antennae, and its three pairs of mouthparts, consisting of paired mandibulae with teeth, maxillae, and palps. Located behind its head, this arthropod's first pair of legs has evolved into a pair of fangs. Although a rare occurrence, when it does bite its victims, the centipede ejects its venom, which is not generally toxic, although it may cause extreme pain to humans.
- Mike Janson and Joyce Pope (consultant editors). The Animal World
- omafra.gov.on.ca image database
- Public Health Image Library (PHIL) Photographs, Illustrations, Multimedia Files