General Features and Characteristics of Super Class - Crustacea:

Crustaceans are mostly aquatic. Head region is different even though head and thoracic are not definitely separate. There are 2 pairs of antennae and at least 3 mouth parts. They have a pair of compound eyes increased on stalks. Have differing numbers of appendages. Appendages are altered for swimming. They have larvae. Usually respiration is through gills that grow out of body walls or limbs. Examples are water flea (Daphnia), prawns, crayfish (Astacus), crabs, lobsters and woodlice. They live and walk on sea floor. Usually body of crustacean is divided in head, thorax and abdomen. Head and thorax are often fused together forming cephalothorax. In head is found specialized appendages while in trunk you find multipurpose ones. Abdomen either has none or decreased appendages except those of tail.

Primitive Appendages:

Consist of the basal joint and 2 jointed branches -biramous or two oared appendage that may both become highly specialized.

The Head:

It always comprise of numerous fused segments. It carries appendages in form of the pair of stalk eyes, 2 pairs of sensory feelers known as antennules and antennae respectively, and 3 pairs of specialized feeding appendages called mandibles, maxillules and maxillae. There may also be extra feeding appendages (maxillipedes) posterior to other. Appendages of head are a good example of specialization of general types to serve different functions and for cooperative use.

Thorax:

Thoracic appendages generally carry out 2 or more functions at once. They are generally highly enhanced but may not be so highly specialized. Biramous appendage may hold original form or be enhanced by expansion or subdivision of base and its 2 branches without being obviously specialized the particular function. The single thoracic appendage may serve to beat and whip in adjoining water in a way that:

a) The food bearing current of water is driven toward mouth.

b) Gaseous exchange between moving blood and current of water over surface of limbs. Such generalized limbs may have additionally to the featherlike divisions, blade-like or leaf-like parts that concurrently serve as paddles and respiratory surfaces. General surface of appendages can be further improved by formation of several chitinous hairs or setae that extend as the fringe from edges of different parts of appendages. Many appendages work together to sift food particles as setae fringes overlap to form grid-like filter. Even as appendage provides many purposes at once, parts of it may be specialized for specific functions. In crustaceans the particular appendage may function mainly in a specific way but in conjunction with other appendages, it roles may vary.

The specialization of limbs has brought about restriction of body in crustaceans. Number of limbs likely to decrease more the specialization. These growths have in turn brought about the huge deal of opportunities for exploiting environment. Shape of crustaceans and variety of limbs differ greatly.

Carapace:

Another result of specialization of limbs is growth of carapace (shield) that produces backwards to cover sides and upper part of thorax. It serves to initiate respiratory and feeding currents and for protection. It is not present in crustaceans. Usually crustaceans exist in separate sexes though for few barnacles are monoecions whereas in few shrimps there is change in sex half way through growth.

Fertilization:

Fertilization occurs as they come out of female to prevent them from solidifying and make sperm penetration impracticable later. So there should be copulation to make sperms accessible as eggs are coming out of oviduct. Once fertilized are enclosed with chitin that make them sticky and get joined to abdominal limbs where they carry on to grow until they hatch in larva. Nature of larva and time it takes them to grow to maturity relies on size of egg. Few crustacean eggs hatch in nauplius larva.

The zoea larva of crabs is larger and has compound eyes. Larvae develop at upper surface of sea and come to bottom after they have developed significantly. The crayfish develop into small forms without larval stage. Crustaceans are varied and can be put into 3 groups depending on type of aquatic life they live.

Classes of Crustaceans:

1) Free-Swimmers (Copepod and Branchiopods):

These are Copepods. They are smallest of crustaceans but most in numbers. They have 3 body parts and characteristic set of appendages. They have major antennae and forked tail. There is single simple median eye. It holds its developing eggs in brood sacs behind them on either side of the abdomen. They liberate eggs as nauplius larvae. They form starting of the food chain. They feed on unicellular plant life and are in turn food to several types of fish. In further growth, some copepods have grown in fish parasites rather than permit later feed on them. These have grown blood sucking tubes from the head procedures for both sucking and attaching themselves to the hosts. Such then loose the sensory and locomotion organs. Their eggs should though be made in large number and grown into free living larvae to allow them discover other hosts. Thus their females produce to large size with much distended bodies laden with abundance eggs. Males remain small and fertile with the locomotor organs to look and fertilize anchor females. Probability of finding the mate are so slim that when in few species male finds the mate, if encounter is made permanent and male becomes the parasite on female. Daphnia, water flea and Artemia brime shrimps are examples of free swimming Branchiopods.

2) Walkers (Decapods):

Primitive crustacean appendage is efficient for swimming for small animals. With the increase for small animals. With the increase in size and extra weight of exoskeleton, crustaceans should have needed inhabit sea floor. Their exoskeleton is consequently converted in heavy armour by impregnation with salts. They give up swimming for walking. The shrimps, lobsters and crabs belong to group. They have common crustacean organization except that branch of one of mouth parts serves as the bailer pulling water respiratory currents forward under side of carapace and over gills. Decapods are also stated for having the gastric mill in its chitin-lined stomach. Comparisons of different adaptations in crustacean appendages:

Panulirus (common shrimp) has two pairs of antennae six pairs of variously specialized mouth parts six pairs thoracic appendages) each having the walking and gill) first three end in clans for gripping. Six pairs of abdominal appendages (all but one biramous) swimming kind. Last appendage joins with last body segment to shape telson lobster. Lobster has all that shrimp has. Except that lobster has held is specialization a bit further. Unlike shrimp, it utilizes only five of six pairs of thoracic appendages for walking. First pair is inflated greatly to form the claw at terminal part, for crushing hard shell of bivalve mollusks on which it fees. Further of these two, one is larger and coarser and serves as the crusher, while other is slenderer and serves as a cutter and nipper. Lobsters may be explained as oversize shrimps.

The crab:

The crab contains everything that lobster contains. Though, crab has wider thoracic and shorter abdomen than lobster or shrimp. Additionally crab has 5 pairs of walking legs articulated to permit walking sideways.

Sessile Crustaceans:

Criripedes and barnacles are sessile. They live tightly attached to rock surface or shell of mollusk, carapace of another crustacean, bottom of ship, floating bottle or on skin of whale. They are anchored by their heads to substrate and have virtually lost senses.

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