how many spines do sea urchins have?
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It varies with species.  Anywhere from a couple of hundred to several thousand.  If you look at the dried test (shell) of a sea urchin, you see that it is covered with tiny rounded bumps, like tiny half beads.  Each of these little smooth round bumps is the "ball" of a ball and socket joint between the test and a spine (the spines are not rigidly attached, then can be moved about). So, if you count the number of these little bumps on the test you can determine the number of spines that urchin had.
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Hi, those measurements will vary according to the species and individuals, and there are many!!\nI suggest two possible sources of measurements: \n1) Papers about morphometry of any stingray (they all belong to the family Myliobatidae - may be considered subfamily Myliobatinae). See below for a good search engine for papers.\n2) Fishbase (link at the bottom), a good reliable source. They have measurement data for some species.\n\nStingrays, in contrast to "real" rays, are not so flat, their heads (especially eyes and spiracles) are raised above the disk, so the thickness will vary according to the place where the measurement is taken, but usually this measurement reflects the maximum thickness. Consider this when you calculate the volume!\nThe shape of the disk also varies according to the genera, so be sure to check it. In particular, the freshwater stingrays (genus Potamotrygon) are almost perfectly circular.\n\nBut well, to make it simpler, Myliobatis are fairly common in the South Atlantic. Here's a paper with measurements for two species:\nhttp://www2.uerj.br/~dbav/equipe/gallo/myliobatis.pdf
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Sea stars or starfish are marine invertebrates belonging to the phylum Echinodermata, class Asteroidea. The names sea star and starfish are also (incorrectly) used for the closely related brittle stars, which make up the class Ophiuroidea. They exhibit a superficially radial symmetry. Starfish typically have five or more "arms" which radiate from an indistinct disk (pentaradial symmetry). In fact, their evolutionary ancestors are believed to have had bilateral symmetry, and sea stars do exhibit some superficial remnant of this body structure.\n\nSea stars do not have movable skeletons, but instead possess a hydraulic water vascular system. The water vascular system has many projections called tube feet, located on the ventral face of the sea star's arms, which function in locomotion and aid with feeding.\n\nDistribution\nThere are about 1,800 living species of sea star, and they occur in all of the Earth's oceans. The greatest variety of sea stars are found in the tropical Indo-Pacific. Areas known for their great diversity include the tropical-temperate regions around Australia,the tropical East Pacific, and the cold-temperate water North Pacific (California to Alaska). Asterias is a common genus found in European waters and on the eastern coast of the United States; Pisaster, along with Dermasterias ("leather star"), are usually found on the western coast. Habitats could range from tropical coral reefs, kelp forests to deep-sea floor, although none of them live within the water column; all species of sea stars found are living as benthos. Echinoderms need a delicate internal balance in their body, no sea stars are found in freshwater environments.\n\nYou could get more information from the link below...
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echinoderms are secondarily radially symmetric whise ancestors were bilaterally symmetric.\nhence, secondary radial symmetry implies that members of a radially symmetrical group came from ancestors with bilateral symmetry.
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http://ceratium.ietc.wwu.edu/IWS/Images/Biota/Animalia/Mollusca/Bivalvia/Psammobiidae/Nuttallia/obscurata\nit's on this page
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Sharks do have backbones, as all vertebrates. Sharks do not have true bone, but their cartilaginous skeleton can be heavily calcified so that their vertebrae have the apperance of "bone" because they look like hard white cylindric spools. You might have seen their vertebrae made into necklaces and such by artisans. \nThis is what a shark trunk vertebra looks like:\nhttp://www.marinebiodiversity.ca/shark/english/porbeagle_vertebra__copy.jpg\nAnd without the neural and haemal arches they look like this: \nhttp://homepage.mac.com/mollet/Cm/BaskingSharkImages/basking%20shark%20vertebra%204.jpg\n\nAnd these are fragments of backbones:\nhttp://www.indonetwork.info/member/77806_backbone2.jpg\n\n(Notice the white color because the cartilage is calcified.)\n\nAlmost all the cartilaginous fish have backbones made of vertebrae (i.e. both sharks and skates/rays); the exception are the chimaeras or ratfish which do not have vertebrae, but retain a notochord (the primitive condition).
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kingdom animalia\nsubphylum invertebrata\nphylum echinodermata\nclass : - asteroidea: starfish\n        - holothuroidea: sea cucumbers\n        - echinoidea: sea cucumbers
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Each of the arms of a starfish has an eye, but they cannot see with it, but they can tell day and night from it.\n\nAlso that means if a starfish has more than 5 legs it has more than 5 eyes.\nSome starfish DO have more than 5 legs\n\nTry this for a site about eyes\nhttp://www.ski.org/Vision/Eyepage/eyeposition.html
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Plesse go to this website \n\nhttp://www.umm.edu/spinecenter/education/anatomy_and_function_of_the_spine.html
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Most starfish reproduce in a method similar to the sponge. The starfish gather in a group (using environmental signals to coordinate the timing), metamorphose into juvenile sea stars and can begin living on the ocean floor.\n\nSea stars are developmentally (embryologically) known as deuterostomes. Since echinoderms and chordates share this same embryological pattern, they are thought to be closely related. Nevertheless, as these creatures are invertebrates and not actually fish, most marine biologists are pushing to completely replace the term starfish with sea star.
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One.\n\nPraying Mantis' have two, one in the head and one in their tail.\n\nHomework question?
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Echinoderms are not segmented, as others have already said.\nAnd the word "echinodermarta" literally means "spiny skin", and they do have skeletal "plates" on their outsides, though in some echinoderms (like sea cucumbers) these are kind of soft.
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According to Wikipedia "Seahorses range in size from 16 mm to 35 cm" - so, knowing the shape of the seahorse, I guess at anywhere between 3 and 7mm.  It is impossible to be exact as the size varies from species to species, and from top to bottom doesn't it?!!
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echinoderms, chordates, hemichordates, protostomes, Ecdysozoa \nLophotrochozoa \nPlatyzoa
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I posted a link below that will give you some information on Sea Monkeys. I know they are a lot smaller than you expect at first anyway. Neat thing for kids to do.
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they can be divided into 2 \nagnatha gnathostomata\nagnatha can be further divided into class cyclostomata\ngnathostomata contains\nchondrichthyes (cartilagenous endoskelton)\nosteichthyes (boney skelton)\namphibia\nreptilia\naves\n& mammals
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Starfish come in various sizes. As small as 1/2 " and I have seen them as large as 10". so the weight depends on the size.  But the average 3" starfish weighs about 3 oz.