Phylum Porifera Animals included in phylum Porifera are parazoans because they do not show the formation of true embryonically derived tissues although they have a number of specific cell types and functional tissues such as pinacoderm These organisms show very simple organization with a rudimentary endoskeleton of spicules and spongin fibers Glass sponge cells are connected in a multinucleated syncytium Although sponges are very simple in organization they perform most of the physiological functions typical of more complex animals Phylum Cnidaria Cnidarians represent a more complex level of organization that Porifera They possess outer and inner tissue layers that sandwich a noncellular mesoglea between them Cnidarians possess a well formed digestive system and carry out extracellular digestion in a digestive cavity that extends through much of the animal The mouth is surrounded by tentacles that contain large numbers of cnidocytes Specialized cells bearing nematocysts used for stinging and capturing prey as well as discouraging predators Cnidarians have separate sexes and many have a lifecycle that involves two distinct morphological forms Medusoid and polypoid at various stages in their life cycles In species with both forms the medusa is the sexual gamete producing stage and the polyp is the asexual stage Cnidarian species include individual or colonial polypoid forms floating colonies or large individual medusa forms sea jellies Superphylum Lophotrochozoa Flatworms Rotifers and Nemerteans This section describes three phyla of relatively simple invertebrates one acoelomate one pseudocoelomate and one eucoelomate Flatworms are acoelomate triploblastic animals They lack circulatory and respiratory systems and have a rudimentary excretory system This digestive system is incomplete in most species and absent in tapeworms There are four traditional groups of flatworms the largely free living turbellarians which include polycladid marine worms and tricladid freshwater species the ectoparasitic monogeneans and the endoparasitic trematodes and cestodes Trematodes have complex life cycles involving a molluscan secondary hast and a primary host in which sexual reproduction takes place Cestodes or tapeworms infect the digestive systems of their vertebrate hosts Rotifers are microscopic multicellular mostly aquatic organisms that are currently under taxonomic revision The group is characterized by the ciliated wheel like corona located on their head Food collected by the corona is passed to another structure unique to this group of organisms the mastax or jawed pharynx The nemerteans are probably simple eucoelomates These ribbon shaped animals also bear a specialized proboscis enclosed within a rhynchocoel The development of a closed circulatory system derived from the coelom is a significant difference seen in this species compared to other phyla described here Alimentary nervous and excretory systems are more developed in the nemerteans than in the flatworms or rotifers Embryonic development of nemertean worms proceeds via a planuliform or trochophore like larval stage Superphylum Lophotrochozoa Molluscs and Annelids Phylum Mollusca is a large group of protostome schizocoelous invertebrates that occupy marine freshwater and terrestrial habitats Mollusks can be divided into seven classes each of which exhibits variations on the basic molluscan body plan Two defining features are the mantle which secretes a protective calcareous shell in many species and the radula a rasping feeding organ found in most classes Some mollusks have evolved a reduced shell and others have no radula The mantle also covers the body and forms a mantle cavity which is quite distinct from the coelomic cavity typically redcuced to the area surrounding the heart kidneys and intestine In aquatic mollusks respiration is facilitated by gills ctenidia in the mantle cavity In terrestrial mollusks the mantle cavity itself serves as an organ of gas exchange Mollusks aso have a muscular foot which is modified in various ways for locomotion or food capture Most mollusks have separate sexes Early development in aquatic species occurs via one or more larval stages including a trochophore larva that precedes a veliger larva in some groups Phylum Annelida includes vermiform segmented animals Segmentation is metameric i e each segment is partitioned internally aswell as externally with various structures repeated in each segment These animals have well developed neuronal circulatory and digestive systems The two major groups of annelids are the polychaetes which have parapodia with multiple bristles and oligochaetes which have no parapodia and fewer bristles or no bristles Oligochaetes which include earthworms and leeches have a specialized band of segments known as a clitellum which secretes a cocoon and protects gametes during reproduction The leeches do not have full internal segmentation Reproductive strategies include separate sexes hermaphroditism and serial hermaphroditism Polychaetes typically have trochophore larvae while the oligochaetes develop more directly Superphylum Ecdysozoa Nematodes and Tardigrades The defining feature of the Ecdysozoa is a collagenous chitinous cuticle that covers the body and the necessity to molt the cuticle periodically during growth Nematodes are roundworms with a pseudocoel body cavity They have a complete digestive system a differentiated nervous system and a rudimentary excretory system The phylum includes free living species of endoparasitic organisms such as Ascaris spp They include dioecious as well as hermaphroditic species Embryonic development proceeds via several larval stages and most adults have a fixed number of cells The tardigrades sometimes called water bears are a widespread group of tiny animals with a segmented cuticle covering the epidermis and four pairs of clawed legs Like the nematodes they are pseudocoelomates and have a fixed number of cells as adults Specialized proteins enable them to enter cryptobiosis a kind of suspended animation during which they can resist a number of adverse environmental conditions Superphylum Ecdysozoa Arthropods Arthropods represent the most successful animal phylum on Earth both in terms of the number of species and the number of individuals As members of the Ecdysozoa all arthropods have a protective chitinous cuticle that must be periodically molted and shed during development or growth Arthropods are characterized by a segmented
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