Review the major evolutionary adaptations which are responsible for the successful radiation and proliferation of the Mullusca, the Annelida, and the Arthropoda into various habitats. Some of the characteristics of molluscs are: bilaterally symmetrical for the most part but in some there is bilateral asymmetry, the body is unsegmented and usually has a definite head, the ventral body wall specialized as a muscular foot which provides a means for locomotion, the dorsal body wall forms the mantle which encloses the mantle cavity, then is modified into gills or a lung, and secretes the shell, but in some the shell is absent, the surface epithelium usually ciliated and has mucous glands and sensory nerve endings, coelom is usually limited to an area around the heart.
The mollusca have a complex digestive system with the anus usually emptying into the mantle cavity. There is also present an open circulatory system (closed in cephalopods) which has a heart, blood vessels, and sinuses. Gas is exchanged by the way of gills, lungs, mantle, or the body surface, kidneys are present. The nervous system is paired cerebral, pleural, pedal, and visceral ganglia, with nerve cords and subepidermal plexus, ganglia centralized in nerve ring in polyplacophorans, gastropods, and cephalopods. The senses of touch, smell, taste, equilibrium, and vision are also well developed.
In some such as the gastropoda and the bivalvia they have very strong shells which provide good protection from predators. The Gastropoda is the largest and most diverse among the molluscs. Some of which are snails, slugs, conchs, limpets, and whelks. They range from a marine environment to the highly evolved air-breathing snails and slugs. Gastropods are usually sluggish because most of them have heavy shells and slow locomotor organs. Although the animals are bilaterally symmetrical, because of torsion(a evolutionary twisting process) the body mass has become asymmetrical.
During this torsion process the anus had gotten placed right over the gills which causes a problem called fouling(waste enters the gills), modern adaptations have allowed for water to be brought in through the left side and out the right carrying waste with it. The bivalvias consist of such animals as clams, scallops, oysters, and shipworms. Most are sedentary suspension feeders that rely on a current provided by the gills to bring food in to the mouth. Most of these animals are marine but they are also found in brackish water and in streams, ponds, and lakes.
The animal bears two shells that have a hinge to allow them to open and close. There locomotion is provided by a slender muscular foot between the valves. In most the foot is used for burrowing but the oysters attach their shells to a surface by secreting cement, and the mussels attach themselves by secreting a many slender byssal threads. There is a incurrent and excurrent that flows across the whole animal with both openings located at the same end. The mouth is located at the opposite end inside the shell which only gets clean, filtered food.
The most interesting of the mollusca is the class cephalopoda. All are marine, and all are active predators. Most swim by expelling water from the mantle cavity through a ventral funnel. To control direction, the funnel can be turned forward or backward and the force of water expulsion determines speed. The nautilus has gas filled chambers that keep the shell upright but can still move fairly fast. The Octopuses can swim using their water jets but are better adapted to crawling around and on rocks and coral.
These animals have a special pigment cell called chromatophores in the skin which by expanding and contracting produce color changes for protection and mating. Strong beaklike jaws can grasp the prey and the radula tears off pieces of flesh. The Annelida or segmented worms, have bilateral symmetry, a nervous system with a double ventral nerve cord and a pair of ganglia with lateral nerves in each metamere, brain, a pair of dorsal cerebral ganglia with connectives to cord, sensory system of tactile organs, taste buds, statocysts, photoreceptor cells, and eyes with lenses.
The blood system is closed and segmentally arranged respiratory pigments often present, the digestive system is complete and not metamerically arranged. The annelid body normally has a head, a segmented body, and a terminal portion bearing the anus. The body wall is made up of strong circular and longitudinal muscles adapted for swimming, crawling, and burrowing and is covered with epidermin and a thin outer layer of nonchitinous cuticle. The hydrostatic (fluid filled) skeleton is very practical because it allows the worms to expand and contract the whole body to allow locomotion.
The Polychaeta are the largest class of annelids. The head is very noticeable and is called the prostomium, which could be retractable, usually bears eyes, antennae, and sensory palps. The atoke is the Anterior, nonreproductive part of a marine polychaete, the Epitoke is the posterior, reproductive part during the mating season. Some of the polychaetes are free-moving pelagic forms, some are active burrowers and crawlers, and some are sedentary, living in tubes or burrows that they rarely leave For the tube and burrow dwellers they are particle feeders and use ciliary or mucoid methods of getting food.
The class of Oligochaeta consist of earthworms and the like, most live in fresh water and take the same long slender form that the earthworm does. They have a poorly developed head and are hermaphroditic (both sex) and exchange sperm during copulation, The worms extend their anterior ends and they are held together via a mucus secreted by the clitellum and by special bentral satae, which penetrate each others bodies in the regions of contact. Seta are used to grip and there are 4 pairs.
When one is lost it is replaced by a new one. Earthworms live underground and come out at night to feed on decayed organic matter. The food and dirt basically passes straight through the digestive track in the body and is filtered by fans. The unwanted dirt is deposited at the end of the worm. The class Hirudinea consist of Leeches numbering over 500 species and are found mostly on fresh water habitats, but a few are marine. They have a fixed number of segments which is usually 34 and have both anterior and posterior suckers.
They do not have a parapodia or any satae. The coelom has become filled with connective tissue and muscle which has an adverse effect on the function of the hydrostatic skeleton. Many leeches are carnivores and get their food by sucking the blood of their pray. Blood sucking leecheds secrete an anticoagulant in their saliva to keep the blood from hardening. The arthropods are found in all types of environments and are very well adapted to each.
Some characteristics include bilateral symmetry, jointed appendages, a head thorax, and abdomen or cephalothorax and abdomen, an exoskeleton of cuticle which is constantly growing and shedding, complete digestive system with mouthparts modified from appendages and adapted for different methods of feeding, circulatory system that is an open system with dorsal contractile heart, arteries and hemocoel, respiration by body surface, gills, tracheae, or book lungs, Sexes usually separate, with paired reproductive organs and ducts which usually internal fertilization, oviparous or ovoviviparous, often with metamorphosis, Parthenogenesis in a few forms, growth with ecdysis. 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. The order Araneae or spiders are Predaceous and feed mostly on insects. The spiders breathe by the use of book lungs or tracheae or both, book lungs are unique to spiders and consist of many parallel air pockets extending into a blood filled chamber. Air enters the chamber through a slit in the body wall.
The spiders use sensory setae which are hair like sensors all over the body, every seta on its surface is used to communicate some information about the surroundings, air currents, or change in the tension in the spider s web. The spiders have a unique excretory system of malpighian tubules that basically recycle waste materials getting rid of only uric acid, by doing this it allows the spider to survive in a very dry climate. The spiders vision is poor and it has eight simple eyes which provide images of only moving objects. The order Acari which are ticks and mites differ from all other arachnids by having their abdomen and cephalothorax completely fused. They are found almost everywhere with over 25,000 species described.
The class Masacostraca is the largest class of crustacea and has great diversity. The trunk of malacostracans usually has eight thoracic and six abdominal somites, each with a pair of appendages. There are many marine and freshwater species including different kinds of crabs, fleas, land bugs, and shrimp. The class insecta which are insects are the most numerous of the arthropods. There are more species of insects than of all other classes of animals combined. Insects have three pairs of legs and usually two pairs of wings. Insects are found everywhere even in the most extreme climate. Their tagamata are the head, thorax, and abdomen. The head usually has compound eyes, a pair of antennae, and usually three ocelli.
The mouth has frequently a labrum, mandibles and maxillae, a labium and a tonguelike hypopharynx, but the type of mouth parts depends on the type of feeding it does. The thorax is composed of three somites: prothorax mesothorax, and metathorax, each eight a pair of legs. And in most the mesothorax and metathorax both have a pair of wings. The legs of insects are modified for a special purpose depending on the environment and lifestyle. Wings come in two different styles: direct flight muscles are attached to a part of the wing itself, and indirect flight muscles are not attached to the wing and cause wing movement by altering the shape of the thorax.