Arthropods Section 1: Features of Arthropods - ppt video online download
1 From the symposium Evolutionary Relationships of Metazoan Phyla: relationships of the coelomate phyla (an- nelids . Relationships of arthropods, annelids, and mollusks .. metazoan evolution: Summary of traditional evi- dence and. closely related to segmented worms, the annelids. There is evolution and relationships of early arthropods. rRNA, strongly simplified and summarized. Fig. An arthropod is an invertebrate animal having an exoskeleton (external .. trilobites, as the evolutionary relationships of this class were unclear. Simplified summary of Budd's "broad-scale" cladogram () then segmentation of arthropods and annelids either has evolved.
There are eight or more classes of molluscs, and many fossil classes, but we will focus on the four most familiar classes of living molluscs.
Arthropods Section 1: Features of Arthropods
Molluscs are protostomes, one of the two main evolutionary pathways taken by the eucoelomate animals. Remember that protostome means "first mouth". The small opening into the embryonic ball of cells that appears early in animal development is called the blastopore. In protostomes, the blastopore becomes the mouth, and the anus appears later on the opposite side. Protostomes like molluscs, annelids, and arthropods develop by spiral cleavage, and their embryonic cells are determinate, the fate of the embryonic cells is fixed very early on in development.
The protostome coelom forms from a split within the mesoderm tissue, so they are sometimes refereed to as schizocoels. Contrast this with the deuterostome animals starfish, chordatesin which the blastopore becomes the anus and the mouth opens elsewhere. Deuterostomes have radial cleavage and their embryonic cells are indeterminate.
Deuterostomes are enterocoels, their coelom forms as out-pockets along the gut. Characteristics of Classes Class Polyplacophora - chitons; sp. They are believed to retain many characteristics of their remote molluscan ancestors.
Chitons have a soft bilaterally symmetric body with a simple tube in a tube body plan, protected by a shell of eight overlapping plates. The body is dorsoventrally flattened, much like their flatworm ancestors. Chitons use their radula to scrape up algae and small animals on rocks and other hard surfaces. When threatened, the chiton creates a vacuum under the shell, almost becoming part of the rock.
Class Bivalvia - mussels, clams, oysters, scallops; 10, sp. Place your fingers on each side and squeeze it together so the two sides meet. Let it fall over on its side - congratulations, you've made a clam! Unlike chitons, bivalves are laterally flattened. The body is enclosed between two valves shellswhich are opened by a hinge ligament. This wedge-shaped body plan is an adaptation for burrowing in soft sand. The shells are closed and held together by a pair of strong muscles called adductor muscles, located at either end of the shell.
It is these adductor muscles that we eat when we eat scallops. Bivalves form a pair of siphons, sometimes formed by folds of the mantle, which let water in incurrent siphon or let water out excurrent siphon. The flow of water is caused by the beating of the cilia that cover the gills. The water current brings in oxygen, food, and gametes, and carries off waste materials. Bivalves are sedentary filter feeders - they don't move around very much. A coating of mucus on their enlarged gills traps small bits of organic matter as the water passes through the bivalve's shell.
The highly mobile trochophore larvae allows these sedentary animals to disperse themselves widely. Class Gastropoda - snails, slugs, limpets, conch, whelks, abalone; 80, sp. This twisting, or torsion, starts during early development.
One side of the larva starts to grow faster than the other, and the snail's body gradually becomes twisted around. Eventually, the visceral mass is rotated a full degrees! The gills in snails are located near the front, a more efficient location for a forward moving animal.
Unlike bivalves, gastropods have a single shell. The twisting of this external shell is actually secondary to the initial twisting of the body mass.
Arthropod - Wikipedia
Torsion may be an adaptation to improve respiration, or to provide better protection against predators. Snails no longer need to clamp down their shell on a hard surface, as chitons do.
They can withdraw into their shell, leaving only a single opening to defend, an opening capped by a shelly plate called an operculum. Torsion also causes a few structural problems. The organs on the right side of the body, such as the gill, nephridium, and the right auricle of the heart, are longer needed, and subsequently disappear. Torsion brings the anus to a rather awkward position directly over the snail's head.
The waste stream must pass out the same hole through which the head emerges bummer! At night, we commonly see many snails with no shells, the slugs.
Molluscs and Annelids
Like all snails, slugs secrete a mucus trail from glands in the foot which helps them move efficiently. Slugs actually have a shell, but the shell is reduced to small plates buried within the outer soft tissues of the animal.
Terrestrial slugs are not especially attractive, but the marine slugs, the nudibranchs, are vividly colored and patterned. Like some flatworms, nudibranchs can eat cnidarians and place the cnidocytes in their own epidermis. Their vivid colors are probably warning coloration.
Like all animals in motion, snails are highly cephalized. Most have a pair of sensory tentacles on the head, and some have primitive eyes on or near these tentacles. Snails also have a radula, a chitinous tongue which they use to scrape algae or animal tissues off the surfaces they glide over. In whelks, the radula is modified as a little drill, which they can use to drill into the shells of other molluscs to feed on them. In many terrestrial snails, the mantle cavity is enriched with blood vessels, and used as a rudimentary lung.
These pulmonate snails can still submerge in water, but must periodically return to the surface in order to breathe. Cephalopods are marine predators, feeding on fish, crustaceans, and other molluscs. They are the only molluscs with entirely closed circulatory systems. With the exception of the Nautilus, cephalopod molluscs lack an external shell. The chambered nautilus enlarges its shell as it grows, living only in the largest outer chamber, and using the spiral of smaller inner chambers to store or release air, so that it can easily rise and fall in the water.
Spiders and Other Arachnids Section 3: Insects and Their Relatives Section 4: Summarize the evolutionary relationship of arthropods and annelids. Identify three subphyla of arthropods. Describe the characteristics of arthropods. Describe how growth occurs in arthropods. Chelicerata, Uniramia, and Crustacea. Compound Eyes Most arthropods have compound eye, which are made up of thousands of individual visual units.
Exoskeleton Arthropods have an exoskeleton made of chitin. Molting Molting is a process when arthropods discard the exoskeleton. Excretion Arthropods have a unique excretory system called Malpighian tubules that efficiently conserves water and eliminates metabolic wastes. Summarize the characteristics of arachnids.
Identify the internal and external characteristics of brown recluse spiders. Compare spiders, ticks, and mites. Identify the health threats posed by some arachnids. Spiders Spiders have a head and a cephalothorax, no antennae, six or eight pairs of simple eyes, a pair of fangs and pedipalps, and four pairs of walking legs. A stinger is located at the end of their abdomen. Mites Mites have body parts that are fused to form an unsegmented body.