ARTHROPODS

Instructor: Almonther I. Alhamedi

The Islamic University of GazaDepartment of Biology

E-mail : . .mhamedi@iugaza edu psWeb page: http:// . . . /site iugaza edu ps mhamedi

CHARACTERISTICS OF PHYLUM ARTHROPODA

1. Bilateral symmetry; metameric body, consisting of head and trunk; head, thorax, and abdomen; or cephalothorax and abdomen

2. Jointed appendages; primitively, one pair to each somite; appendages often modified for specialized functions

3. Exoskeleton of cuticle containing protein, lipid, chitin, and often calcium carbonate secreted by underlying epidermis and shed (molted) at intervals.

4. Complex muscular system, with exoskeleton for attachment, striated muscles for rapid actions, smooth muscles for visceral organs; no cilia.

5. Reduced coelom in adult; most of body cavity consisting of hemocoel (sinuses, or spaces, in the tissues) filled with blood

6. Complete digestive system; mouthparts modified from appendages and adapted for different methods of feeding

CHARACTERISTICS OF PHYLUM ARTHROPODA

7. Open circulatory system, with dorsal contractile heart, arteries, and blood sinuses

8. Respiration by body surface, gills, tracheae (air tubes),

9. Paired excretory glands called coxal, antennal, or maxillary glands present in some, homologous to metameric nephridial system of annelids; some with other excretory organs, called malpighian tubules

10. Nervous system of annelid plan, with dorsal brain connected by a ring around the gullet to a double nerve chain of ventral ganglia; fusion of ganglia in some species; well developed sensory organs

11. Sexes usually separate, with paired reproductive organs and ducts; usually internal fertilization; oviparous or ovoviviparous; often with metamorphosis; parthenogenesis in some

SUBPHYLUM CRUSTACEA

General Nature of a Crustacean crustaceans are the dominant arthropods in the sea. The bodies of crustaceans are covered with a secreted cuticle

composed of chitin, protein, and calcareous material. Crustaceans are the only arthropods with two pairs of

antennae. In addition to two pairs of antennae and a pair of mandibles,

crustaceans have two pairs of maxillae on the head, followed by a pair of appendages on each body segment or somite.

In some crustaceans not all somites bear appendages. All appendages, except perhaps the first antennae, are primitively biramous (two main branches).

Crustaceans are specialized for life in water, and most possess gills to obtain oxygen.

The appendages are specialized for swimming, crawling, attaching to other animals, mating, and feeding.

SUBPHYLUM CRUSTACEA The largest group of crustaceans is class

Malacostraca, which includes lobsters, crabs, shrimps, beach hoppers, sow bugs, and many others.

SUBPHYLUM CRUSTACEA These show a surprisingly constant arrangement of body

segments and tagmata, This typical body plan has a head of five (six embryonically) fused somites, a thorax of eight somites, and an abdomen of six somites (seven in a few species).

At the anterior end is a non-segmented rostrum and at the posterior end is a non-segmented telson, which with the last abdominal somite and its uropods forms a tail fan in many forms.

In many crustaceans the dorsal cuticle of the head may extend posteriorly and around the sides of the animal to cover or be fused with some or all of the thoracic and abdominal somites. This covering is called a carapace.

In decapods (including lobsters, shrimp, crabs, and others), the carapace covers the entire cephalothorax but not the abdomen.

CHARACTERISTICS Head: 2 pairs antennae, pair mandibles (jaws), 2 pairs

of maxillae (handle food) Usually 1 pair appendages for each segment Tagmata either cephalothorax & abdomen or head (5

fused somites), thorax (8 somites), and abdomen (6 somites)

Rostrum—anterior end Telson & uropods form tail—posterior end

External structure of crayfishes. A, Dorsal view. B, Ventral view.

Appendages of a crayfish showing how they have become modified from the basic biramous plan, as found in a swimmeret. Protopod, pink; endopod, purple; exopod, yellow.

EXTERNAL FEATURES Dorsal covering is carapace; may cover most of body

or just cephalothorax Somites without carapace have tergum (dorsal) or

sternum (ventral) plates which are heavy and have more calcareous deposits

Cuticle made of protein, chitin, and calcareous deposits.

Joints soft & thin for flexibility Telson analogous to pygidium (posterior body part)

APPENDAGES Specialization of appendages; all show serial

homology (derived from same biramous type) All variations of biramous (two jointed) plan with a

basal portion called the protopod (1-2 joints) and a side portion called an exopod (1-several joints), and a mid portion called a endopod (1-several joints)

Joints will have further modifications called endites or exites

BODY CAVITY Metamerism but no intersepta No coelom, this is lost during development; blood-

filled hemocoel instead with no peritineal lining.

BODY CAVITY

MUSCULAR SYSTEM

Striated muscle forms most of body Antagonistic pairs; flexors and extensors Flexors in abdomen allow crayfish to swim backward Muscles alongside stomach control mandibles

RESPIRATORY SYSTEM

Small crustaceans exchange gases through cuticle on thin areas of body like legs.

Large crustaceans use feather-like gills. The gills may be in thorax cavity or on thoracic legs.

CIRCULATORY SYSTEM

Open circulatory system: no veins to separate blood from interstitial fluid

Hemolymph (blood) leaves dorsal heart (1 chamber) by arteries but washes through tissue sinuses; goes to sternal sinus and then on to gills for gas exchange before returning through pericardial sinus

Hemolymph colorless; hemocyanin and/or hemoglobin are respiratory pigments

NERVOUS AND SENSORY SYSTEMS More fusion of ganglia than other arthropods Supra-esophageal ganglia connect eyes and antennae Neurons join this to sub-esophageal ganglion that

controls mouth, appendages, esophagus, and antennal glands

Double ventral nerve cord with ganglion/somite to control appendages

Eyes-compound and made of many units (ommatidia) (one of the optical unit in compound eyes in crustaceans) restricted to mosaic vision in bright light; in dim light sees continuous image

Statocysts located at base of 1st antennae Tactile hairs on, mouthparts, and telson

Copepods are extremely abundant and important in the plankton. They use their mouthparts to filter out or capture food. Many planktonic species keep from sinking by using their enlarged first pair of antennae ( Fig. 7.30 ) to swim. Many species are parasitic, some being so simplified that they look like little more than small bags of tissue.

The structurally simplest crustaceans, like this planktonic copepod, tend to be small and have appendages that are similar to each other—that is, less specialized. One exception here is the first antenna, which is specialized for swimming. All appendages are paired, but the ventral view shows only those on one side.

Barnacles are filter feeders that usually live attached to surfaces, including living surfaces like whales and crabs. Adults are sessile and attach directly (acorn) or by a stalk (goose) Carapace surrounds body and is secreted as calcareous plates, Head reduced, abdomen absent, legs (cirri) long with setae for filter feeding

The plates on the upper surface open to allow the feathery filtering appendages (cirri), which are actually the legs, to sweep the water.

(a) Barnacles conceal a crustacean body beneath thick plates. They lie on their backs and use their legs to filter feed. Note that barnacles are hermaphrodites.

(b) The thatched barnacle (Tetraclita squamosa) from the Pacific coast of North America.

Amphipods are small crustaceans with a curved body that is flattened sideways.

The head and tail typically curve downward, and the appendages are specialized according to function. Beach hoppers, common in shore debris, are strong jumpers.

Other amphipods crawl among seaweeds. Burrowing in the skin of whales (as whale lice) and living as part of the plankton are some other lifestyles of this large, mostly marine group of over 5,000 species.

beach hopper (Orchestoidea)

Isopods are found in many of the same environments as amphipods. They are about the same size as amphipods.

the body is dorsoventrally flattened and thus has a flat back. Pill bugs are common land isopods that are similar to many marine species. Fish lice (no relation to lice of birds and land mammals, which are insects) and other isopods are parasites of fishes.

The sea louse, or sea roach (Ligia oceanica) , is neither a louse nor a roach but an isopod. It feeds mainly on decaying seaweeds carried in to shore by the waves. Some isopods live as parasites on fishes.

Krill, or euphausiids, are planktonic, shrimp-like crustaceans. The head is fused with some of the body segments to form a distinctive carapace that covers the anterior half of the body like armor. Most krill are filter feeders that feed on diatoms and other plankton. Krill are extremely common in polar waters, aggregating in gigantic schools of billions of individuals. They are an almost exclusive food source for many Antarctic whales, penguins, and fishes.

Krill (Euphausia superba) form dense schools in Antarctic waters and were thought to be potential human food.

Shrimps, Lobsters, and Crabs With around 10,000 species,the decapods (means“10 legs”) are the largest group of crustaceans

Decapods feature five pairs of legs, or pereopods, the first of which is heavier and usually has claws used for feeding and defense.

Decapods also have three pairs of maxillipeds, which are closer to the mouth, turned forward, and specialized to sort out food and push it toward the mouth. Maxillipeds are used as filtering devices in decapods that eat small food particles.

The carapace is well developed and encloses the part of the body known as the cephalothorax. The rest of the body is called the abdomen.

Limulus polyphemus is a horseshoe crab

Pleuroncodes planipes, also called the lobster krill or tuna crab, is a pelagic crab that has been suggested as a potential source of human food.

DECAPODA

The American lobster ( Homarus americanus ) illustrates the basic body plan of decapod crustaceans.

Hermit crabs, which are not true crabs, are also scavengers. They hide their long, soft abdomens in empty gastropod shells. Some hermit crabs cover their shells with sea anemones or sponges for added protection and camouflage.

In the true crabs the abdomen is small and tucked under the compact and typically broad cephalothorax. The abdomen is visible as a flat, V-shaped plate in males; in females it is expanded and U -shaped for carrying eggs .

Hermit crab (Dardanus sp.) with sea anemones living on its shell. The hermit crab is protected by the nematocysts on the sea anemone’s tentacles, and the sea anemones get to eat food particles that are let loose when the hermit crab feeds.

Crabs have broader cephalothorax and reduced abdomen compared to crayfish or lobsters