Arthropod central nervous system closely resembles that of annelids though former illustrates higher degree of cephalization. Astacus contains double ventral nerve cords whose strands are separate over most of their length but are joined by segmental ganglia which exhibit some degree of fusion. Brain forms the ring round anterior end of alimentary canal; it is created by cerebral ganglia (that lie anterior to mouth) joined to suboesophageal ganglion by pair of circumoesophageal commissures. Remaining thoracic and abdominal ganglia are separate structures in segments 9 to 19. Astacus contains well-developed system of giant fibres which are similar with those of annelids that is two median fibres extend from brain to telson, and two laterals arise segmentally and are not joined to brain. Astacus contains very well-developed sense organs. It contains compound eyes of type found among arthropods, that are capable of detecting not only different light intensities but of forming images. These are large and prominent, and made up of about 2,500 units called ommatidia (sin. ommatidium), each pointing in a slightly different direction. Balancing organs or statocysts are very significant in aquatic crustaceans like Astacus. These are in form of sacs, that open to outside through tiny pore at base of antennules. Every sac is lined with sensory hairs and has statolith, bunch of sand grains which move around hitting against sensory hairs, causing impulses to be sent to brain. Setae which react to touch and chemoreceptors happen on mouthparts and on antennules and antennae. Antennae are mainly significant as they can be held forwards or bent posteriorly to increase information ahead and behind as animal moves.
Reproductive system and reproduction in Astacus:
Sexes are separate in Astacus, like in most crustaceans. Reproductive organs are pair of testes or ovaries connected to form Y-shaped structure; they give off two anterior canals which extend to sides of pericardial space, and single posterior canal. In female two short ducts open near bases of second pair of walking legs. Walls of oviducts are liable for secreting protective chitinous shell over eggs. In male sperm ducts are extended and extremely convoluted, and open at bases of fourth walking legs. Glandular lining secretes the sticky substance that surrounds sperm while in sperm ducts. Sperm have no tails and are star-shaped with several spines. Mating occurs shortly after female has experienced her pre-adult moult. The male uses first two pairs of modified abdominal appendages to place and stick spermatophores onto posterior sterna and pleopods of female.
External fertilization occurs after partners have separated. Female lies on her back with abdomen curved forwards to form the space, roofed by abdominal terga with sides formed by long setae on lateral margins of terga. Eggs are discharged in space, and moved about by current produced by beating pleopods. Fertilized eggs are cemented to special setae on pleopods by fluid produced by specialized glands that forms protective coat round each egg. Development occurs within yolky egg. Eggs are carried by female until they hatch as miniature adults, lacking few abdominal appendages. They stay attached to female's pleopods by particularly modified, hooked chelipeds until they are completely grown. Free-swimming planktonic larva happens in most marine and also some freshwater crustaceans, the earliest basic kind being nauplius (plural nauplii). It contains only 3 pairs of appendages:
2 pairs of antennae and a pair of mandibles; it lacks trunk segmentation and contains single median or nauplius eye. When first eight pairs of trunk appendages are free of carapace, larva in higher malacostracans is known as zoea. Postlarva may be fairly similar to adult in general appearance. Basic developmental pattern of nauplius, zoea (or its equivalent), and postlarva is very often altered.
Significance of crustaceans:
Uses other than for food:
Crustaceans are important primary and secondary consumers in many aquatic systems, and are especially crucial in limnetic waters (the lighted surface water in a lake with no rooted aquatic plants). Some species support direct fisheries and aquiculture for human food or fish bait, important fish and wildlife foods. Some may help control unwanted aquatic vegetation. Many are good environmental indicators for metals, acid rain, global warming, etc.
Subphylum Uniramia (Latin: unus, one; ramo, branch)
Subphylum Uniramia is distinguished by individuals with uniramous (single-branched) appendages (as name implies), one pair of antennae and 2 pairs of mouthparts (single pair each of mandibles and maxillae). Uniramia are mainly terrestrial, though several species have secondarily invaded freshwater, a few have reinvaded the marine environment and one group has the ability to fly. Uniramia includes millipedes, centipedes and insects.
Diagnostic and special characteristics of subphylum Uniramia:
Bilaterally symmetrical; < 1 mm-35 cm long arthropods varying in body shape from extremely elongate to almost spherical. Through, straight gut with no digestive diverticula. Body separated in two regions, a head and three or four appendage-bearing segments, and trunk bearing pairs of walking legs; in one class, trunk includes series of up to 350 relatively uniform segments, most of which bear walking legs. Appendages are uniramous, those of head comprise one pair each of antennae, mandibles, and maxillae, and in some groups second pair of maxillae, those of trunk all form functional or altered walking legs; with no chelicerae or chelate limbs. Head with lateral ocelli, often organized in compound eyes; at times also with median ocelli. Trunk, but not head, externally segmented. With a fat body in haemocoel, frequently closely related with gut.
Phylum Onychophora (Velvet worms):
These are terrestrial caterpillar-like animals; approx 12cm long. They have combination of arthropod and annelid characters and have been explained as missing link between both groups; they have also been termed as living fossils. They happen in damp forests beneath logs of woods, leaves and beneath bark of rotten logs. They are mainly nocturnal and avoid light. They are mainly found in Tasmania, New Guinea, New Zealand, South Africa, Australia and Chile. The best known is Peripatus.
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