marine bio midterms

• Nematodes (phylum Nematoda), commonly known as roundworms, are hardly ever seen, but their numbers in sediments, particularly those rich in organic matter, can be staggering.
• Nematodes are perfectly adapted to live in sediments or in other organisms. They are mostly small, with slender and cylindrical bodies that are typically pointed at both ends
• The adults of Anisakis and a few related nematodes inhabit the intestine of seals and dolphins.
• In nematodes a layer of muscles in the tough but flexible body wall pushes and squeezes against the fluid, which acts as a hydrostatic skeleton that provides support and aids in locomotion.
• In terms of number of species, the arrow worms, or chaetognaths (phylum Chaetognatha), rank among the smallest animal phyla.
• The arrow worms are nevertheless one of the most common and important members of the plankton. They are almost transparent, streamlined, with fish-like fins and tail. The head has eyes, grasping spines, and teeth. Total length ranges from a few millimeters to 10 cm (up to 4 in)
• Arrow worms are voracious carnivores with efficient sensory structures to detect their prey. They prey on small crustaceans, the eggs and larvae of fishes and other animals, other arrow worms, and practically anything else that is small. They spend most of their time motionless in the water but will swim in rapid, darting movements to grab their prey.
• A large group of perhaps as many as 20,000 species, the segmented worms, or annelids (phylum Annelida), includes earthworms and many marine worms.
• The annelids the body consists of a series of similar compartments, or segments, a condition known as segmentation.
• Segmentation can be clearly seen in the rings of the familiar earthworm. The gut goes through all the segments and lies in a cavity known as a coelom.
• When longitudinal muscles contract they shorten the segments, lengthening them when the muscles relax, whereas contraction and relaxation of circular muscles reduces and increases segment diameter, respectively.
• Almost all marine annelids are polychaetes (class Polychaeta), which are common and important in many environments. Each of the body segments of most polychaetes has a pair of flattened extensions, or parapodia, which are provided with stiff and sometimes sharp bristles, or setae.
• Like all annelids, polychaetes have a circulatory system that transports nutrients, oxygen, and carbon dioxide. Circulating blood always remains within distinct blood vessels, making it a closed circulatory system.
• In polychaetes, muscular contraction of vessels helps in the circulation of blood. Wastes from the coelom are removed by a pair of excretory organs in each segment.
• In small animals, oxygen—essential in the release of energy through respiration—can easily move from the water across the body wall to all the tissues. In the larger and relatively more active polychaetes, however, obtaining enough oxygen from the water is a potential problem. Polychaetes have solved this problem by evolving gills on the parapodia or elsewhere.
• The gills are thin-walled extensions of the body wall that have many blood vessels called capillaries, which allow for the easy absorption of oxygen.
• The life history of many polychaetes involves a planktonic larval stage known as the trochophore, which has a band of cilia around the body.
• Fanworms, or feather-duster worms, use feathery tentacles covered with cilia to capture, sort, and transport particles.
• Serpulids (Serpula) and spirorbids (Spirorbis), also suspension feeders, extend feather-like tentacles from calcium carbonate tubes they build on rocks and other surfaces.
• In the tropical Pacific the bodies of the Palolo worm (Eunice) periodically break off, and the posterior half swims up to the surface to spawn. This behavior, known as swarming, is timed in some areas with the phases of the moon, reaching its peak just after full moon.
• Beard worms, also known as pogonophorans, and vestimentiferans are highly specialized polychaetes. They lack a mouth and gut.
• Bloodsucking leeches (class Hirudinea) live mostly in fresh water, but marine species can be found attached to marine fishes and invertebrates.
• Long regarded as a separate phylum, echiurans are now considered annelids (class Echiura). They look like soft, externally unsegmented sausages buried in the mud or in coral.
• Often called peanut worms, the sipunculans (phylum Sipuncula) have soft, unsegmented bodies with a coelom. Some biologists consider them to be annelids.
• The gastropods (class Gastropoda), are the largest, most common, and most varied group of molluscs.
• The body of molluscs is covered by a mantle, a thin layer of tissue that secretes the shell.
• Snails, clams, octopuses, and other familiar forms are members of the phylum Mollusca.
• A feature unique to molluscs is the radula, a ribbon of small teeth that is used to feed, usually by rasping food from surfaces.
• The radula is made largely of chitin, a highly resistant carbohydrate also found in other invertebrates. Gas exchange is through paired gills.
• Nudibranchs, or sea slugs, are gastropods that have lost the shell altogether. Colorful branches of the gut or exposed gills make nudibranchs among the most beautiful of all marine animals.