Google Scholar. Adoutte, A. Animal evolution: The end of the intermediate taxa? Trends in Genetics, 15 3 , — CrossRef Google Scholar. Arthur, W. The origin of animal body plans: a study in evolutionary developmental biology. Cambridge: Cambridge University Press. Berg, L. Introductory botany: Plants, people, and the environment. Nelson Education. Cole; 2 nd edition. Budd, G. The morphology of Opabinia regalis and the reconstruction of the arthropod stem-group.
Lethaia, 29 1 , 1— Thomas Eds. Arthropod body-plan evolution in the Cambrian with an example from anomalocaridid muscle. Lethaia, 31 3 , — Does evolution in body patterning genes drive morphological change — Or vice versa? BioEssays, 21 4 , — A critical reappraisal of the fossil record of the bilaterian phyla. Biological Reviews, 75 2 , — David, B. Major events in the evolution of echinoderms viewed by the light of embryology. Sea spiders are known as pycnogonids pycno - means closely packed and gonid refers to gonidia, which is a group of asexually reproductive cells.
Sea spiders are in the class Pycnogonida. They are not true spiders, or even arachnids. Sea spiders have long legs with relatively small bodies Fig. They are found across the ocean, with over species. Most are small even as small as 1 mm and live in relatively shallow depths. However some deep and cold water species grow to over 90 cm. The subphylum Myriapoda is represented by land-dwelling arthropods such as centipedes and millipedes. This group contains over 13, species. There are no known marine myriapod species.
They have a single pair of antennae and mouthparts roughly similar to those found in chelicerates. The main distinguishing feature of myriapods is the multiple jointed legs that extend from their elongated worm-liked bodies. The subphylum name comes from Greek root words meaning countless feet. Myriapods have between 10 to legs. The class Insecta is another primarily terrestrial group of arthropods, although insects have been reported from nearly all environments with the exception of deep-sea habitats.
Insects are the most diverse members of the subphylum Hexapoda Fig. Hexapods have a distinct body plan, which includes three large sections: a head, thorax, abdomen with three pairs of thoracic legs and one pair of antennae Fig. Hexapods have three sets of jaws called mandibles, maxillae, and labium Fig. Insects breathe by taking in air through spiracles into trachea tubes. Unlike in the arachnids, these tubes do not terminate at book lungs, but branch into smaller networks of tubes called tracheoles that branch directly into the tissues of the insect for gas exchange Fig.
There is no active pumping of air, but any small movement in the insect body leads to airflow throughout the trachea. Along with their close relatives the centipedes and millipedes, insects have appendages that are unbranched.
Their bodies are developed into three distinct segments of head, thorax and abdomen. The wings usually two pairs are contained on the thorax segment along with three pairs of legs. Insect wings and legs are modified in many ways depending upon the lifestyle of the insect. In most insects, though, the legs include sensory receptors. Other sensory receptors are found on the body, and insects have compound eyes and light sensing ocelli on their heads. Most marine arthropods belong to the subphylum called Crustacea Fig.
Some species of shrimp, called krill, spend their lives as plankton, drifting in the surface waters of the open oceans.
Some species of crustaceans, such as crayfish, are common in freshwater lakes and streams; a few—isopods and pill bugs, for example—live on land. Many crustaceans are used as food, both by larger animals and by humans. While most crustaceans are mobile, one exception is the barnacle.
Barnacles are free swimming as larvae, but when they metamorphose, the head reduces and is cemented to the substrate with a strong chemical glue. The barnacle secretes a hard protective shell around itself and filter feeds from inside this shell, using elongated appendages to collect food particles.
A sea anemone and its symbiotic anemone fish. Three comb jellies ctenophores. Comb jellies resemble tiny hot air ballons the size of a walnut or smaller with eight rows of fused cilia comb plates extending down the sides. They propel themselves mouth first by the eight rows of comb plates. Ctenophores superficially resemble miniature medusae phylum Cnidaria ; however, most medusae arise asexually from a polyp generation and ctenophores have no polyp stage in their life cycle.
Tentacles extending from the mouth contain "glue cells" or colloblasts containing spiral threads which snare small fish and crustaceans with a gluelike material. With the exception of one species, ctenophores do not have the stinging organelles nematocysts of jellyfish and sea anemones.
During the day, ctenophores flash prismatically as their ciliary plates refract light; at night they are often bioluminescent, glowing like little lamps. Is this a 1 wet pajama draw string, 2 a long noodle, or 3 a human tapeworm? The answer is 3 , a 20 inch 50 cm human tapeworm. The small head or scolex from which the segments progottids arise is clearly visible in the photograph. The presence of a scolex is good evidence that the entire worm has been discharged from the host's intestine.
Each proglottid contains a complete male and female reproductive system. In fact, one proglottid may contain literally thousands of eggs. Magnified view 30X of the human tapeworm shown in the above photograph showing the anterior end or scolex. The scolex bears four circular suckers which firmly anchor the tapeworm to the host's intestinal wall.
The scolex produces proglottids by budding, which gradually enlarge as more segments are formed. Tardigrades belong to a remarkable phylum of minute multicellular animals.
They are adapted to extreme conditions, some of which are more severe than any earth environment. Does their origin defy natural selection? Lateral side view of the exoskeleton of an aquatic tardigrade Hypsibius sp. There are 4 pairs of stout, stumpy legs, each tipped with several slender claws.
The name "water bear" refers to its deliberate "pawing" sort of locomotion. Thomas Huxley, English naturalist and good friend of Charles Darwin, gave tardigrades this name in Photographed with a Sony W digital camera through an Olympus laboratory grade compound microscope x magnification. An aquatic tardigrade of the genus Hypsibius. Its length is approximately micrometers microns , about the same length as the hair follicle mite Demodex brevis.
It is much smaller than a grain of common table salt NaCl. The image was enhanced with Photoshop to bring out detail of the claws. Magnification x. M4V file. Mesozoic rock from Arizona showing numerous brachiopods, evidence of a shallow sea along a continental shelf.
An articulate hinged brachiopod. Gastropods shells cowries and cone shells from the south Pacific island of Moorea and Tetiaroa Atoll in French Polynesia. The lower right shell is from a land snail. The common garden snail Helix aspersa. This snail was introduced into California in the s by European immigrants for use as food.
It turned out to be a poor substitute for the edible snail escargot served in French restaurants Helix pomatia. It has since become well-established and a major nuisance in California gardens. Homeowners spend more money on eradicating this creature than all other invetebrate pests combined.
The head has two pairs of retractile tentacles. The longer, upper tentacles each have an eye at the tip. The shorter, lower tentacles bear chemosensory organs equivalent to taste and smell.
The ventral side contains a mouth with a rasping tongue radula for feeding. This snail is hermaphroditic bisexual with both male and female sex organs. Although capable of self fertilization, the normal method is cross fertilization between two mating partners.
The snail moves by the gliding action of its large muscular foot. Mucus secreted from slime glands on the foot provides a lubricant to facilitate movement, and leaves a silvery slime trail. Snail mucus is used in "snail cream" cosmetics. A gentleman at the University of Utah once quipped that if we destroyed every ounce of soil and rock, air and ocean, you could still see the earth from outerspace - the outline made purely from nematodes. Haldane's quip was rehearsed.
He said it in many different contexts, like any originator of a good quip should. He also said the "two brothers, eight cousins" quip many times. I thought the "two brothers or eight cousins" quote was Fisher. Is there an on-line repository of great quotes from scientists? I can only find collections of quotes from all sorts of people. I think there are a few described species that are obligate parasites of fungi, too.
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