Introduction to Subphylum Chelicerata:
In the last study unit, we learnt that there were three classes in the subphylum Chelicerata. Brief description of the three classes is given below.
Class Merostomata (horseshoe crabs):
Horseshoe crabs aren't associated to true crabs at all. There are just 4 living species of horseshoe crabs living in world's oceans, and they haven't changed much in form in last 350-400 million years. They are found only in waters off eastern North America, Southeast Asia, and Indonesia.
Class Pycnogonida (sea spiders):
Though commonly known as sea spiders, they aren't related to arachnids, but like arachnids, several of them have four pairs of long walking legs. All species are marine, with the fossil record extending back approx 500 million years. They are frequently parasitic on corals, jellyfish, and their relatives, as young, and predators of slow-moving animals as adults.
Class Arachnida (spiders, ticks, mites, scorpions):
Arachnids all share the distinctive body plan which unites them and separates them from other two groups of chelicerates. There are approx 11 orders of arachnids alive today, having approx 74,000 described species.
Subphylum Crustacea (crabs, prawns, shrimps, lobsters):
Crustaceans can be considered as highly successful arthropods, and comprise shrimps, lobsters and crabs, which are of gastronomic interest, and numerous smaller forms. Majority of crustaceans are marine species; approx 97% of all marine arthropods are crustaceans, but few live in freshwater. Many species of crustaceans live above high tide line on beaches but the relatively small number are terrestrial. They haven't been very successful as terrestrial animals as they have retained typically aquatic physiology and are thus limited to damp environments. Many crustaceans live as parasites. Crustaceans can usually be differentiated from other groups of Arthropods by possession of 2 pairs of antennae, and by presence of biramous limbs. Crustaceans are about completely aquatic, with only few hundreds of approx 40,000 known species living on land; many species have free-swimming planktonic larvae.
The main crustacean characteristics:
The nine classes of subphylum Crustacea are explained below:
Class Cephalocarida: Small, blind, less than 4mm long; detritus feeding, bottom dwelling. Body separated in head, thorax and abdomen without cepahalothorax or carapace. Generally hermaphrodite with paired ovaries and testes sharing common duct.
Class Branchiopoda: The diverse group of mostly freshwater crustaceans. Small to vestigial head appendages (except antennae); trunk segments aren't fused to head; trunk segments with series of alike limbs which decrease towards posterior; last few segments lack limbs.
Class Ostracoda: Very small, generally marine, few terrestrial; less than 1mm long, hardly ever approaching 2mm. Short oval body surrounded in bivalved frequently calcareous shell formed by carapace; such as some bivalve molluscs, shell has adductor muscles for shutting shell, but unlike them, shell is shed at each moult.
Class Mystacocarida: Minute, less than 1 mm long, elongate, no pigment, marine; distinguished by head which is divided into small anterior and large posterior portion, and trunk of ten segments; first one bears a maxilliped even though it is not fused to the head. Head appendages are large and used in locomotion, trunk appendages are reduced, or are missing; the telson bears a large, pincer-like furca.
Class Copepoda: Dominant members of marine plankton and to some extent of that in fresh water as well. Approx 25% of all species are parasitic on animals from sponges to whales. Many species are less than 2 mm long; though, one free-living species is approx 2 cm and ectoparasite approx 0.3 m long.
Class Branchiura: Small, less than 3 cm long. Periodic ectoparasites of marine and freshwater fish. They are dorso-ventrally flattened, having cephalothorax of head and first thoracic segment, pereon of three segments, and bi-lobed unsegmented abdomen; cephalothorax and, in some, much of pereon is covered by a large, flat carapace, and bears a pair of compound eyes. The eggs are attached to substratum or vegetation.
Cirripedia: Most extremely altered of Crustacea, being either sessile or dwellers in other organisms in parasitic manner. They are efficiently headless, mostly do not have abdomen, and there is little or no evident segmentation. Order Rhizocephala resemble the bracket fungus, with network of fine tubes spreading through all tissues of host and external sac having gonads. Order Ascothoracica parasitize cnidarians and echinoderms and are least specialized anatomically.
Class Malacostraca: By far largest class, with approx 23,000 species, and, it has greatest diversity of body form than all other classes; just one of its 16 orders, Decapoda, comprises such differed organisms as crayfish, crabs, shrimps and hermitcrabs.
Class Remipedia: Represented by the single blind species only explained from marine cave, very little is known of its biology. Body is smallish, lengthen and translucent, less than 3 cm long. A pair of rod-like processes in front of antennules.
Astacus, a representative crustacean (Class Malacostraca):
External covering is formed by segmented exoskeleton that is extremely thick and hard except at arthrodial/articular membranes; this, as we saw earlier, is secreted by single layer of epidermal cells. Surface layer, epicuticle, is thin and made up of tanned lipoprotein, providing impermeability to entire structure. Endo-cuticle is thick and made of three layers: outer pigmented layer of sclerotized protein, middle layer of deposited calcium carbonate and inner uncalcified layer.
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