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.
Take the chitons body and twist it into a spiral, and you have created a snail. 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. 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. 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. Stretch the chiton's body vertically, and carve the foot into several tentacles - you've made a cephalopod! 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.
The ventral foot of the chiton becomes a posterior foot, divided into a highly modified set of tentacles , 10 in the squid, 8 in the octopus. These tentacles are all equipped with large sucker discs, which can be used for defense, as well as for capturing and manipulating prey.
The mouth is equipped with poison glands. Cephalopods stun or kill their prey with toxic saliva, carry it to their mouth with their tentacles, and then tear the prey apart with their strong beak and radula. Male cephalopods use a modified tentacle to place sperm into the female's mantle cavity during reproduction.
Giant squid can reach a length of over 60 feet. Giant octopi have been seen in the Sea of Japan with arms up to 45 feet long! Even though they are rarely seen, we know that giant squid exist, because of the titanic battles in the ocean depths between giant squid and the sperm whales that eat them. Sucker scars on the sides of whales can be used to estimate the size of these sea monsters.
The mantle cavity of cephalopods is modified as an escape mechanism. Cephalopods can forcefully expel water from the mantle cavity by quickly closing their mantle and jetting away to a safe place. Cephalopods also squirt dark ink to hide their escape. Octopi also crawl about the ocean floor, using their tentacles. The actively swimming squid uses jets of water from the mantle cavity to propel itself through the sea.
Because of the great length of the squid's body, it uses a single large nerve cell to send the escape message from its brain down to its lower body. This nerve cell is so large that a narrow glass tube can be inserted inside the slender axon to permit experiments and observations on nervous conduction. Study of these giant nerves gave us our first insights into how nerve cells conducted electrical signals. Cephalopods are highly cephalized, with large, complex brains capable of primitive problem solving, and some very advanced sensory organs.
The eye of the octopus is very elaborate, with a retina and basic structure very similar to the eyes of vertebrates. It is a marvelous example of convergent evolution. Examine slides of the radula of a snail , and think about how this strange tongue operates. Remember you are looking down at it from above. Observe the live mussels.
Can you see the siphons at work? The siphons pump water into and out of the clam for filter feeding and for respiration. Observe the live snails. Note their cephalization. Can you see the tentacles? Watch how they use their foot to glide along the glass. If you watch carefully, you may see them extrude their radula to scrape algae or small critters from the glass. When disturbed, they retreat into their shell and close the door, a round disc called an operculum.
Observe the slug as it crawls along. Look on the underside of the glass and watch the smooth waves of muscular contraction visible on the base of the foot. Examine the clam. Find its anterior, posterior, ventral and dorsal sides. Observe the incurrent and excurrent siphons. In this species, the siphons may be visible as spaces along one edge of the mantle. In other species, like the razor clam, these siphons are prominent tubes sticking out of the shell.
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. Polychaete worms have paired, unjointed limbs called parapodia on each segment used for locomotion and breathing.
Beneath the cuticle there are two layers of muscle, one running around its circumference circular and one running the length of the worm longitudinal. Annelids have a true coelom in which organs are distributed and bathed in coelomic fluid. Annelids possess a well-developed complete digestive system with specialized organs: mouth, muscular pharynx, esophagus, and crop.
A cross-sectional view of a body segment of an earthworm is shown in [Figure 6] ; each segment is limited by a membrane that divides the body cavity into compartments.
Gas exchange occurs across the moist body surface. Annelids have a well-developed nervous system with two ventral nerve cords and a nerve ring of fused ganglia present around the pharynx. Annelids may be either monoecious with permanent gonads as in earthworms and leeches or dioecious with temporary or seasonal gonads as in polychaetes.
This video and animation provides a close-up look at annelid anatomy. Phylum Annelida includes the classes Polychaeta and Clitellata [Figure 7] ; the latter contains subclasses Oligochaeta, Hirudinoidea, and Branchiobdellida.
Earthworms are the most abundant members of the subclass Oligochaeta, distinguished by the presence of the clitellum , a ring structure in the skin that secretes mucus to bind mating individuals and forms a protective cocoon for the eggs. The chaetae of polychaetes are also arranged within fleshy, flat, paired appendages on each segment called parapodia.
The subclass Hirudinoidea includes leeches. Significant differences between leeches and other annelids include the development of suckers at the anterior and posterior ends, and the absence of chaetae.
Additionally, the segmentation of the body wall may not correspond to internal segmentation of the coelomic cavity. This adaptation may allow leeches to swell when ingesting blood from host vertebrates. Include several examples of organisms in division.
Have flagellated spores zoospores Chytridium fruiting spores Zygomycetes Live as parasites or symbionts with animals. Form mycorrhizae Fast growing molds, Rhizopus stolonifer black bread mold , pilobolus. Ascomycetes Sac fungi. Saprobes, plant parasites, have symbiotic relationships with algae lichens. Produce sexual spores asci. Have more extensive dikaryotic stage. Morals, carbon fungus, truffles.
Decomposers of wood and other plant material. Outline the major characteristics Campbell uses to define an animal. List an hypothesis for the origin of animals.
The ancestor was probably a flagellated protest; which was probably related to choanoflagellates. Describe the two forms of symmetry of the Eumetazoa. What is the significance of cephalization as an evolutionary trend? A trend toward the concentration of sensory equipment on the anterior region. Also development of the central nervous system.
How do the germ layers of Radiata and the other Eumetazoa differ? They are diploblastic having 2 germ layers. Define the following terms and describe their significance in classifying animals.
Acoelomates lack a coelom. Coelomates possess a true coelom, a body cavity completely lined with tissue c. Protostomes development begins with spiral, determinate cleavage. The coelom forms from splits in the mesoderm. The mouth forms in the blastopore. Deuterostomes development is characterized by radial, indeterminate cleavage. The mouth forms from a secondary opening. The anus develops from the blastopore. Determines developmental fate of embryonic cells early on.
Each cell contains the capacity to develop into a complete embryo. List a number of the major differences between the Protostomes and Deuterostomes. Protostomes - development begins with spiral, determinate cleavage. The cleavage mouth forms from the blastopore. The ceolom forms from mesodermal outpocketings of the archenteron. Label the stages of early embryonic development of animal. Use definitions from Question 25 to supply the details in your chart.
Include examples of organisms in each division. Porifera Simple; sessile animals that lack true tissues. They live as suspension feeders; trapping particles that pass through the internal channels of their bodies. Cnidaria Share a distinctive body pattern that includes a gastrovascular cavity with a single opening that serves as both mouth and anus. Ctenophora Diploblastic. They also have a unique method for catching prey.
Comb jellies 4. They have no body cavity or organs for circulation. Rotifers Specialized organ systems, including an alimentary canal digestive tract. They feed on microorganisms suspended in water. Rotifers 6. Nucleic acid and protein analyses have informed the construction of the modern phylogenetic animal tree. Evolutionary trees can be made by the determination of sequence information of similar genes in different organisms. Sequences that are similar to each other frequently are considered to have less time to diverge, while less similar sequences have more evolutionary time to diverge.
The evolutionary tree is created by aligning sequences and having each branch length proportional to the amino acid differences of the sequences. Furthermore, by assigning a constant mutation rate to a sequence and performing a sequence alignment, it is possible to calculate the approximate time when the sequence of interest diverged into monophyletic groups. Phlyogenetic tree of life : Advances in molecular biology and analysis of polymeric molecules such as DNA, RNA, and proteins have contributed to the development of phylogenetic trees.
Sequence alignments can be performed on a variety of sequences. For constructing an evolutionary tree from proteins, for example, the sequences are aligned and then compared. This is best supported by research of Dr.
Carl Woese that was conducted in the late s. Since the ribosomes are critical to the function of living organisms, they are not easily changed through the process of evolution. Taking advantage of this fact, Dr. Woese compared the minuscule differences in the sequences of ribosomes among a great array of bacteria and showed that they were not all related. For example, a previously-classified group of animals called lophophorates, which included brachiopods and bryozoans, were long-thought to be primitive deuterostomes.
Extensive molecular analysis using rRNA data found these animals to be protostomes, more closely related to annelids and mollusks. This discovery allowed for the distinction of the protostome clade: the lophotrochozoans. Molecular data have also shed light on some differences within the lophotrochozoan group. Some scientists believe that the phyla Platyhelminthes and Rotifera within this group should actually belong to their own group of protostomes termed Platyzoa.
Molecular research similar to the discoveries that brought about the distinction of the lophotrochozoan clade has also revealed a dramatic rearrangement of the relationships between mollusks, annelids, arthropods, and nematodes; a new ecdysozoan clade was formed. Due to morphological similarities in their segmented body types, annelids and arthropods were once thought to be closely related.
However, molecular evidence has revealed that arthropods are actually more closely related to nematodes, now comprising the ecdysozoan clade, and annelids are more closely related to mollusks, brachiopods, and other phyla in the lophotrochozoan clade.
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