Marine invertebrates
As the name suggests, marine invertebrates lack any mineralized axial endoskeleton, i.e. the vertebral column, and some have evolved a rigid shell, test or exoskeleton for protection and/or locomotion, while others rely on internal fluid pressure to support their bodies.
Animals are multicellular eukaryotes,[note 1] and are distinguished from plants, algae, and fungi by lacking cell walls.
[6] Apart from Dickinsonia, the earliest widely accepted animal fossils are the rather modern-looking cnidarians (the group that includes jellyfish, sea anemones and Hydra), possibly from around 580 Ma[7] The Ediacara biota, which flourished for the last 40 million years before the start of the Cambrian,[8] were the first animals more than a very few centimetres long.
[15] The small shelly fauna are a very mixed collection of fossils found between the Late Ediacaran and Middle Cambrian periods.
The earliest, Cloudina, shows signs of successful defense against predation and may indicate the start of an evolutionary arms race.
[16] In the 1970s, there was already a debate about whether the emergence of the modern phyla was "explosive" or gradual but hidden by the shortage of Precambrian animal fossils.
[16] A re-analysis of fossils from the Burgess Shale lagerstätte increased interest in the issue when it revealed animals, such as Opabinia, which did not fit into any known phylum.
[17] Later discoveries of similar animals and the development of new theoretical approaches led to the conclusion that many of the "weird wonders" were evolutionary "aunts" or "cousins" of modern groups[18]—for example that Opabinia was a member of the lobopods, a group which includes the ancestors of the arthropods, and that it may have been closely related to the modern tardigrades.
[21][22]: 33 A body plan refers to a blueprint which describes the shape or morphology of an organism, such as its symmetry, segmentation and the disposition of its appendages.
Invertebrate sea life includes the following groups, some of which are phyla: Arthropods total about 1,113,000 described extant species, molluscs about 85,000 and chordates about 52,000.
They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of cells.
Instead, most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes.
The shapes of their bodies are adapted for maximal efficiency of water flow through the central cavity, where it deposits nutrients, and leaves through a hole called the osculum.
A few species of sponge that live in food-poor environments have become carnivores that prey mainly on small crustaceans.
Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick.
They have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes.
[34] It is easy to care for in the laboratory and a protocol has been developed which can yield large numbers of embryos on a daily basis.
[50] They are ubiquitous in marine, freshwater and terrestrial environments, where they often outnumber other animals in both individual and species counts.
The phylum contains about 7,000 living species,[54] making it the second-largest grouping of deuterostomes (a superphylum), after the chordates (which include the vertebrates, such as birds, fishes, mammals, and reptiles).
Aside from the hard-to-classify Arkarua (a Precambrian animal with echinoderm-like pentamerous radial symmetry), the first definitive members of the phylum appeared near the start of the Cambrian.
The majority of species still live in the oceans, from the seashores to the abyssal zone, but some form a significant part of the freshwater fauna and the terrestrial ecosystems.
Firstly, it has a muscular cloak called a mantle covering its viscera and containing a significant cavity used for breathing and excretion.
The simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur.
[67]: 284–291 [67]: 298–300 [69][70] Good evidence exists for the appearance of marine gastropods, cephalopods and bivalves in the Cambrian period 538.8 to 486.85 million years ago.
However, the evolutionary history both of molluscs' emergence from the ancestral Lophotrochozoa and of their diversification into the well-known living and fossil forms are still subjects of vigorous debate among scientists.
Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many setae (bristles) that project through their cuticles.
Arthropods' methods of reproduction and development are diverse; all terrestrial species use internal fertilization, but this is often by indirect transfer of the sperm via an appendage or the ground, rather than by direct injection.
The group is generally regarded as monophyletic, and many analyses support the placement of arthropods with cycloneuralians (or their constituent clades) in a superphylum Ecdysozoa.
There are a number of marine invertebrates that use minerals that are present in the sea in such minute quantities that they were undetectable until the advent of spectroscopy.
Vanadium is concentrated by some tunicates for use in their blood cells to a level ten million times that of the surrounding seawater.
