Distance and maximum-likelihood analyses of 18S rRNA gene sequences from major spiralian taxa suggest a sister relationship between annelids and molluscs and provide a clear resolution within the major groups of the spiralians. The parsimonious tree based on molecular data, however, indicates a sister relationship of the Annelida and Bivalvia, and an earlier divergence of the Gastropoda than the Annelida-Bivalvia clade.
To test further hypotheses on the phylogenetic relationships among annelids, molluscs, and arthropods, and the ingroup relationships within the major spiralian taxa, we combine the molecular and morphological data sets and subject the combined data matrix to parsimony analysis.
Digestive System continued Organic food particles and inorganic soil particles go through long tube-like esophagus into a round organ called the crop. The crop stores the food until it is forced through the gizzard. Digestive System continued The gizzard contracts and expands causing grains of sand and food to rub together. Food is finally digested in the intestine.
Intestine Food Particles. Digestive System continued Digested food is absorbed by the blood circulating through the intestinal wall. Wastes pass out of the anus. Digestive System continued Extra Information As worms feed they loosen soil, soil would be very compacted and hard with out worms.
Circulatory System Blood Flows through a set of tubes or vessels. After digestion, blood carries the products of digestion to all body cells. Circulatory System continued Earthworm have no true heart. Wastes are passed to the outside of the body through nephridia.
They are a simplistic version of our kidneys. Nervous System and Sense Organs Segment number three contains a pair of fused ganglia made up of nerve cells; function as the brain.
Nerves Ventral Nerve Chord branch from these ganglia to control the muscles and to receive stimuli from each segment. Nervous System and Sense Organs continued Earthworms have no eyes or ears but are sensitive to light and vibrations. Muscular System have an outer and inner layer of circular muscles in the body wall. Also have longitudinal muscles.
Both of these together, along with setae, are use in locomotion. Reproductive System Are hermaphroditic but do not self-fertilize. Sperm is produces in two pair of testes and stored in sacs called seminal vesicles. A sperm duct carries the sperm to the male genital pore. Reproductive System continued Eggs are produced in the ovaries and mature in the body cavity. Reproductive System continued Sperm is passed from one worm to another on the spot called the seminal receptacles.
The clittellum secretes a slime ring which contains the eggs. The eggs are then in turn fertilized by the sperm. Phylum: Mollusca Mollusks are soft-bodied animals that usually have an internal or external shell. Segmentation Divisions of body sections. Earthworm has about segments.
Slugs, snails, and animal that. Mollusca and Annelida. Mollusks Section Soft-bodied Animals Snails, slugs, oysters, clams, scallops, octopuses, squid A larval stage called a trochophore Second largest. Clams, Oysters, etc. They are found in marine and freshwater habitats. As the name suggests, bivalves are enclosed in a pair of shells or valves that are hinged at the dorsal side.
The body is flattened on the sides. They feed by filtering particles from water and a radula is absent. They exchange gases using a pair of ctenidia, and excretion and osmoregulation are carried out by a pair of nephridia. In some species, the posterior edges of the mantle may fuse to form two siphons that inhale and exhale water. This property is commercially exploited to produce pearls.
Watch animations of clams and mussels feeding to understand more about bivalves. Gastropods include shell-bearing species as well as species with a reduced shell. These animals are asymmetrical and usually present a coiled shell [Figure 3].
The visceral mass in the shelled species is characteristically twisted and the foot is modified for crawling. Most gastropods bear a head with tentacles that support eyes. A complex radula is used to scrape food particles from the substrate. The mantle cavity encloses the ctenidia as well as a pair of nephridia. Cephalopods include shelled and reduced-shell groups.
They display vivid coloration, typically seen in squids and octopuses, which is used for camouflage. The ability of some octopuses to rapidly adjust their colors to mimic a background pattern or to startle a predator is one of the more awe-inspiring feats of these animals.
All animals in this class are predators and have beak-like jaws. All cephalopods have a well-developed nervous system, complex eyes, and a closed circulatory system. The foot is lobed and developed into tentacles and a funnel, which is used for locomotion. Suckers are present on the tentacles in octopuses and squid. Ctenidia are enclosed in a large mantle cavity and are serviced by large blood vessels, each with its own heart. Cephalopods [Figure 4] are able to move quickly via jet propulsion by contracting the mantle cavity to forcefully eject a stream of water.
Cephalopods have separate sexes, and the females of some species care for the eggs for an extended period of time. Although the shell is much reduced and internal in squid and cuttlefish, and absent altogether in octopus, nautilus live inside a spiral, multi-chambered shell that is filled with gas or water to regulate buoyancy.
Tooth shells have a radula and a foot modified into tentacles, each with a bulbous end that catches and manipulates prey [Figure 5]. Annelida Phylum Annelida are segmented worms found in marine, terrestrial, and freshwater habitats, but the presence of water or humidity is a critical factor for their survival in terrestrial habitats. The name of the phylum is derived from the Latin word annellus , which means a small ring.
Approximately 16, species have been described. The phylum includes earthworms, polychaete worms, and leeches. Like mollusks, annelids exhibit protostomic development. Annelids are bilaterally symmetrical and have a worm-like appearance. Their particular segmented body plan results in repetition of internal and external features in each body segment.
This type of body plan is called metamerism. The evolutionary benefit of such a body plan is thought to be the capacity it allows for the evolution of independent modifications in different segments that perform different functions. The overall body can then be divided into head, body, and tail. The skin of annelids is protected by a cuticle that is thinner than the cuticle of the ecdysozoans and does not need to be molted for growth. Chitinous hairlike extensions, anchored in the skin and projecting from the cuticle, called chaetae , are present in every segment in most groups.
The chaetae are a defining character of annelids.
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