Arthropoda

ARTHROPODA

(Greek.arthos = jointed + podos = foot)
The phylum arthropoda contains the greatest majority of all the known animals (around 1 mil species). Most of these species are also very abundant as individuals. It includes the crustaceans ( Crustacea), insects (Insecta), the spiders, scorpions, ticks and their equals (Arachnida), the centipedes (Chilopoda), and the milipedes (Diplopoda). One of the most important phylums ecologically, because it dominates all terrestrial and aquatic ecosystems in numbers of species, individuals or both. Also because a major share of the energy passing through these systems passes through arthropod bodies.
All arthropods are externally segmented in a varying degree. Their limbs are jointed and are differentiated for special purposes. The whole of the body is covered with a hard exoskeleton of chitin and in some cases strengthened by calcium carbonate. Because the exoskeleton cannot expand, arthropods periodically have to shed their exoskeletons, a process called moulting, and then they can expand rapidly. Immediately after moulting, arthropods are soft and vulnerable, untill a new layer of chitin is deposited and becomes hard. Insecta and Araneae are very ill represented in the ocean. Sea spiders and Crustaceans on the other hand are by far the most diverse groups of arthropods in the sea. Their bodies normally comprise of a fused head and thorax (cephalothorax) plus a separate. They have various and numerous appendages adapted for walking, swimming, jumping, flying, respiration and reproduction. Characteristically there exists two pairs of antennae and at least some of the limbs divide into two branches said to be "biramous". The exoskeleton may for a shield that covers, protects and unites various segments of the body. Crustaceans have intricate life cycles. Most have multiple larval stages which are planktonic and are widely dispersed by the currents before they metamorphose into miniature versions of the adults.

Characteristics:

1. Symmetry bilateral. Triploblastic. Body Usually segmented and joined externally. The head, thorax and abdomen are variously distinct or fused (the head segments are always fused).

2. There are one pair of appendages per segment or less (each with few to many hinge joints and containing opposed sets of muscles. The limbs are variously differentiated and sometimes reduced in number or parts, but rarely lacking.

3. A hardened exoskeleton is present containing chitin that is secreted by the epidermis. The exoskeleton is moulted at regular intervals.

4. The muscles are striated and often complex. They are usually capable of quick reactions.

5. A complete digestive canal is present. Different mouth parts, with lateral jaws, that are adapted for chewing or for sucking. The digestive tract ends in an anus.

6. The circulatory system is open (lacunar). The hart is at the dorsal side distributing blood via arteries to tissues and organs. It returns through the body spaces (hemocoel) to the heart. The body cavity is reduced in size.

7. Respiration is either by gills, tracheae (air ducts), book lungs or through the body surface.

8. Excretion occurs either by coxal or green glands, or by 2 to many Malpighian tubules that are joined directly to the gut.

9. A nervous system is present with paired dorsal gangalia over mouth and connectives to a pair of ventral nerve cords (with a gangalia or gangalia concentrated in each segment. The sensory organs include antennae and sensory hairs (tactile and chemoreseptive) simple and compound eyes, auditory organs (Insecta), and statocysts (Crustaceae).

10. The sexes are usually seperate and the male and female animals are often unlike. Fertilization mostly occurs internal. Eggs with much yolk, in shells are formed. The animals are either oviparous or ovoviviparous, cleavage usually superficial. The life cycle usually include one to several larval stages and a gradual or abrupt metamorphosis to adult form. Parthenogenesis in some crustaceans and insects.

HEXAPODA

Characteristics:

1. Head, thorax and abdomen distinct.
The head consists of one pair of antennae, mouth parts for chewing, sucking, or lapping (consisting of mandibels, maxillae, and a labium - fused 2nd maxillae).
The thorax cosists of three segments with a pair of jointed legs on each and usually two (or one or no) pairs of wings.
The abdomen consists of eleven or fewer segments with the terminal parts modified as genitalia.

2. A digestive canal with a fore-, mid- and hindgut is present. The mouth contains salivary glands.

3. The heart is slender with lateral ostia and an anterior aorta (no capilaries or veins). The body spaces are a hemocoel (coelom reduced).

4. Respiration Respiration occurs via branced cuticule-lined tracheae that carry oxygen from paired spiracels on sides of thorax and abdomen directly to the tissues (exept with Collembola). Some aquatic forms poses tracheal or blood gills.

5. Excretion by two or more fine Malpighian tubules attached to the anterior end of the hindgut (except Collembola)

6. A nervous system of supra- and subesophageal ganglia is present connecting to a double ventral nerve cord, with one pair or fewer ganglia per segment. The sense organs usually include simple and compound eyes, chemoreceptors for smell on antennae and for taste about the mouth and various tactile hairs. Some poses means of sound production and reseption. No statocysts.

7. The sexes are sperate.Poseses gonads of multiple tubules with one median duct posteriorly. Fertilization occurs internally. Eggs with much yolk and protecting shells. Cleavage is superficial exept with Collembola. Development is either with several moults and direct, or with multiple nymphal stages and gradual metamorphosis, or with several larval stages, a pupa and complete metamorphosis to adult form. Parthenogenesis in aphids, thrips, gallwasps etc.

COLLEMBOLA

Springtails

Springtails are minute to 5mm long. They are coloured or white. They have antennae present which are 4 to 6 jointed. They poses chewing mouth parts that are folded inwards. They lack wings, compound eyes, malpighian tubules and there are usually no tracheae. It poseses an abdomen of 6 partialy fused segments. Most of th pecies leap by the action of a ventral springing organ (furcula) on the 4th abdominal segment, when it is released by the hook (hamula) on the 3rd segment. A ventral tube on the irst segment recieves a sticky secretion from a gland behind the labium and that is how an animal adheres to a smooth surface. 10 Families, 2000 species.
Anurida Maritima Intertidal habitat.

INSECTA

Insects

Insects have a distinct head, a thorax bearing three pairs of legs, and usually one or two pairs of wings, and a limbless, segmented abdomen.

Diagramatic, lateral view of a winged insect.

Anterior surface of the head of an insect.

They are abundant in terrestrial and freshwater habitats, but remarkably few occur at sea. Some are associated with rotting seaweeds on sandy beaches, notably the kelp flies. On rocky shores the long worm-like larvae of midges graze on sea weeds, and minute black water-beetles are found about the high tide mark.

CHELICERATA

Characteristics:

1. Body usually of distinct cephalothorax (prosoma) and abdomen(copisthosoma). Typically with six pairs of jointed appendages: chelicerae, pedipalpi, and four pairs of legs all on the cephalotorax. Lacking antennae or mandibels.

2. The mouth parts and digestive canals are mainly suited for sucking, some contain poison glands.

3. Respiration ocurs through book lungs, tracheae, or book gills.

4. Excretion by paired Malpighian tubules or coxal glands, or both.

5. A nervous system is present including dorsal ganlia - a brain - and ventral nerve cord having paired ganglia or else concentrated anteriorly. The eyes are usually simple and paired. Tactile hairs or bristles present on body.

6. The sexes are mostly seperate. There are one or two sex openings on the anterior of the abdomen. Fertilization usually occurs internally. Mainly oviparous development direct or through a larval stage.

7. Chiefly terrestrial and solitary. Either free-living and predaceous or parasitic.

MEROSTOMATA

Horseshoe crabs

This class is the only marine, gill-bearing chelicerates. Dorsally the carapace bears a pair of compound eyes, and dorsally six pairs of appendages. The abdominal segments are fused together and end in a ling spikelike telson. The underside of the abdomen carries five pairs of book gills (comprising of a large number of thin plates -lamellae - protected by a flaplike cover.

A ventral view of a horseshoe crab.

A dorsolateral view of a horseshoe crab.

Despite appearances hey are harmless. They crawl over or push in mud in shallow water, swim upside down with the use of gills as paddles. The telson comes in handy to flip the animal right side up when it has flipped over.
Horseshoe crabs are omnivores and scavengers with a diet including soft-bodied invertebrates and algae they encounter while ploughing through the ground.
During the reproductive period horseshoe crabs congregate in shallow waters to mate. Eggs are fertilized as they are deposited into shallow depressions in the sand. There they are covered and left. A larva hatches and swims and burrows in the sand. It gradully aquires the adult form over a succession of molts.

PYCNOGONIDA

Sea spiders

Although sea spiders poses four pairs of long legs, just like the true spiders the two groups are not related. Sea spiders have a head that is armed with a tubular proboscis and often a pair of jointed, senosory palps plus a pair of pincer-like feeding appendages (chelifers), a trunk consisting of four segments, and it also poseses a conical abdomen. A pair of slender ovigerous legs usually hang below the head and are used by the males to cary the spherical egg masses. Most of the sea spider species are carnivorous (feeding on hydroids, anemones or bryozoans).

ARACHNIDA

ARANEAE

Spiders

The bodies of true spiders comprise of an anterior prosoma, that is the same as a fused head and thorax, and a posterior opisthosoma (abdomen). The prosoma houses a pair of fowardly directed venomous fangs and a pair of leg-like feelers (pedipals), which are used by the males for the transfer of sperms, and also the four pairs of elongate walking legs. In the abdomen the lungs are setled together with the reproductive organs and silk glands (typical of this group).

The internal anatomy of a spider.

Although spider are mostly terestrial there are a few species living on the coasts in the intertidal zones.

CRUSTACEA

Characteristics:

1. Head of five fused segments including two pairs of antennae, one lateral pair of mandibels for chewing, and two pairs of maxillae.
A thorax consisting of 2 to 60 segments distinct or variously fused.
Abdominal segments are usually distinct with telson at the end.
Often with a carapace covering the head and parts of the thorax as a dorsal shield or as two lateral valves. The appendages are variously modified and some are usually biramous.

2. Respiration via gills (rarely by body surface). Pseudotracheae on pleupods of some land isopods.

3. Excretion by antennal or maxillary glands. No Malpighian tubules.

4. The sexes are seperate (except in Cirripedia an a couple of others). The sex openings are paired. The eggs are often carried by the female. There are usually one or more larval stages.

OSTRACODA

Seed shrimps

Seed shrimps are small crustaceans where their heads and short, oval bodies are completly enclosed by a hard bivalved carapace, hinged along the centre of the back. The larger forms of 1-4mm usually have a hatch at the front of each shell through which the hairy antennae protrude into the exterior, while the smaller species are very smoothly oval. Some ostracods are planktonic, but most species bore, plough or crawl through the suface layers of mud and sand, propelled by their antennae. The group includes carnivores, herbivores and scavengers.

COPEPODA

Copepods

A generalized copepod (dorsal view).

A group of very small creatures of various body forms. Most species are free living, but some are parisitic. Many of these parasites have degenerate, sac-like or worm-like bodies, with trailing egg-strings.
Centroppages brachiatus is an example of the order Calanoidia which includes copepods with tapering, cylindrical bodies and very large antennae projecting at right angles to its head, and a single central eye. They are usually the the most noticable part of plankton samples.
Porcellidium sp. is an example of the bottom-dwelling copepods of the order Harpacticoida that have short antennae and include broad, flattenad forms, which coloniate seaweeds and the elongated forms which live in the spaces between the sand-grains on coarse sandy beaches.

CIRRIPEDIA

Barnacles

The adults are sessile, attached to rocks or they are parasitc. They attach by means of a cement gland on the first antennae. The carapace is modified into a mantle that surrounds the body. It usually has limpy plates. It has 6 or fewer pairs of slender and bristly biramous appedages behind the mouth, used for the gathering of food.

Diagram of the internal structure of a sessile barnacle(lateral view).

A cluster of sessile barnacles.

MALACOSTRACA

A generalized malacostracan crustacean

Lateral view of body.

A thoracic appendage.

Ventral view of the mouthparts.

This class includes the lobsters, crayfishes, crabs, pill bugs, beach hoppers, and mantis shrimps. The body is typically divided into 19 segments ( 5 for the head, 8 for the thorax, 6 for the abdomen). The head is fused to one or more thoracic somites. The mouth parts include mandibels and maxillae with palps. Usually a carapace is present. The abdomen has appendages. 18,000 species.

PHYLLOCARDIA

Nebalia

A small primitive group of crustaceans. The thorax and part of the abdomen are enclosed by a large, bivavled carapace. The eyes are stalked and seperated by a characteristic, visor-like rostrum that protrudes from the front of the head. The abdomen ends in a pair of elongate, rappering appendages.

STOMATOPODA

Mantis shrimps

They have a head with two movable segments bearing eyes and antennules. They have enlalged antennae. Gills are present on their abdominal appendages. The second pair of thoracic appendages are modified for raporal feeding. The carapace is small. They are up to 2,5cm in size.

A mantis shrimp.

It dwells in the sand or crevices. Around 200 species.

TANAIDACEA

Tanaids

Tanaids are small, cylindrical crustaceans with unstalked eyes. The anterior thoracic segments are fused to the head and is covered by a short carapace. The other six thorasic segments remain free. The first pair of legs bears strong nippers (distinctive feature of this group), leaving six pairs of legs free for walking. Branched pleopods lie beneath some or all of the abdominal segments. the body ends in a rounded pleotelsonwith a pair of long segmented uropods. Tanaid are generally sand dwelling animals in depths of up to 3,650 m (They live in mud or tubes). They either filterfeed or they grasp peices of detritus or small organisims from the vicinity of the burrow entrance. Eggs are carried in a brood pouch under the body and develop directly into the adult form.

CUMACEA

Cumaceans

The cumaceans are small, distinctively shaped crustaceans with an enlarged, swollen carapace with two anterior extensions (often joined over head) and a very narrow, elongat abdomen ending in a pair of slender uropods. They usually burrow in surface sand or in mud where they either filter food or gather organic matter from the sand, but they may emerge to swim in the water column.

ISOPODA

Isopods

Pill bugs. The thoracic appendages, except the first, are all the same. The body is usually depressed dorsoventrally and lacks a carapace. The abdomen is short and partially or fully fused. They live among plants or under rocks, some are terrestial and others are parasitic on fish and crustaceans. There are about 4,000 species.

AMPHIPODA

Amphipods

Sand hoppers. Each of the thoracic and abdominal appendages are in at least two different functional groups. The body is often laterally compressed and lacks a carapace. The abdomen is flexed ventrally between the third and the fourth somites. The telson is usually distinct.