The beauty of a living thing is not the atoms that go into it, but the way those atoms are put together.
Idea Transcript
Bell Pettigrew Museum of Natural History Interpretative Panels Text: Dr Iain Matthews Design: Steve Smart & Cavan Convery
A University of St Andrews Development Fund Project School of Biology http://biology.st-andrews.ac.uk
14:1 Chelicerata P hy l u m
Chelicerata
S u b p hy l u m Super class Class
Body Plan: • Bilaterally symmetrical • Triploblastic • Haemocoelic cavity (coelom also present?) and through gut • Protostome • Segmented, with complete chitonous exoskeleton
The chelicerates are stiff-bodied, three-layered animals, with a spacious haemocoel, and a simple through gut bearing diverticula. Contained within the larger grouping Arthropoda, the chelicerates have a hard chitonous exoskeleton, with simple segmentally arranged jointed appendages. Their two-part body is divided into an anterior prosoma which has 6 leg-bearing segments and a posterior opisthosoma with has no legs and shows external segmentation (up to 12 segments). They differ from other arthropod phyla in a number of respects, most obviously in the absence of true ‘jaws’. The main mouthparts are formed from one pair of chelicerae and one pair of pedipalps that are adapted for sucking liquid foods and are incapable of chewing or biting. In addition there are no antennae present.
found in shallow marine habitats.
Chelicerates show relatively limited structural diversity and cover only a limited size range. The majority of species are liquid-feeding carnivores (especially preying on insects), but the diverse use of silks and poisons has helped to extend their variation. Extant forms include: marine scavengers (e.g. king crabs), terrestrial predators (either active hunters such as scorpions and whip spiders or trappers such as most true spiders), ectoparasites (e.g. most mites and ticks) and omnivores (e.g. harvestmen).
Classification within Chelicerata
The phylum is split into three classes: Class: Merostomata The king crabs, or horseshoe crabs , were a dominant group until the Permian but are represented by only four extant species. They have prominent gill-bearing appendages on the opisthosoma, and are
Gut Endoderm Body Cavity Mesoderm Ectoderm Exoskeleton
Class: Arachnida With over 62,000 species the arachnids, including the spiders , scorpions , mites and ticks are by far the largest of the classes. They lack appendages on the opisthosoma and the prosoma is partly covered by a carapace. They have four pairs of walking legs and lack compound eyes. Class: Pycnogonida Around 1000 species of sea spiders have been identified, although some zoologists consider the pycnogonids to be a separate arthropod phylum and dispute the position within Chelicerata. They possess chelicerae but can have between 4 and 6 pairs of walking legs and lack a carapace.
Spider, scorpions, ticks and mites Ticks and mites are important as the carriers of a range of diseases (e.g. Lyme disease and Louping Ill) of humans and livestock. Spiders feed by sucking fluids from their victims. However they have evolved to eat a wide range of prey including fish and birds .
Horseshoe crabs were once far more common than they are today. However the majority of species died out during the Permian and today only 4 species are left. The sea spiders are especially common in polar waters,they feed on sponges and bryozoans and can reach up to 80 cm in diameter.