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Unformatted text preview: 470 Somewhere around 700 million years ago (mya), the earli-est animal life forms arose. They were marine organisms, much like the modern sponges. Like sponges, these pri-mordial animals existed as a loose aggregation of cells, bathed in seawater. Each cell of a marine sponge is bathed in seawater, but maintains an intracellular ion composition different from that of seawater, using ion pumps and active transport to create and maintain the electrochemical gra-dients that drive transport and synthetic processes. Over the next 150 million years, evolution led to impor-tant changes in how animal tissues were organized. An early milestone was the formation of tissue layers. Next came the capacity to produce a specialized external tissue layer using cells that were interconnected in ways that lim-ited the passage of seawater into the body. The formation of this epithelial tissue provided barriers between the ex-ternal world and internal fluids, resulting in the establish-ment of an extracellular fluid that was separate from the external environment. Animals differ in their ability to con-trol the osmotic and ionic nature of this extracellular fluid. Though sponges lack true tissues, other ancient ma-rine invertebrates, such as flatworms (Platyhelminthes), Ion and Water Balance C H A P T E R 1 0 8140606_CH10_p470-525.qxd 10/11/08 8:47 PM Page 470 P e a r s o n L i g S l u t N F R O D b 471 possess true tissues and can create an extracellular fluid that is physically separated from seawater. Yet in these simple animals, the extracellular fluid contained within the tissues is identical to seawater in its ionic and osmotic properties. However, multiple lineages of animals have evolved mechanisms that provide much greater control over the properties of their extracellular fluids. The ability to control extracellular fluid composition allowed animals to invade brackish water, freshwater, and even land. Major changes in osmoregulation and ionoregulation occurred in the evolution of the chordates. Like many sim-ple marine invertebrates, the earliest chordates were ma-rine organisms that had little control over the nature of their extracellular fluid composition. The extracellular fluid of the hagfish, a marine agnathan (jawless fish), is similar to seawater, although it is somewhat reduced in the concentrations of Ca 2 , Mg 2 , and SO 4 2 . Cartilaginous fish control the ion composition of the extracellular fluid, but the osmolarity is close to that of seawater. Bony fish regulate both the ionic and osmotic profile of their extra-cellular fluids. This ability to control internal ionic and os-motic properties was essential to the diversification of bony fish, which now occupy almost every aquatic and semiaquatic niche on the planet, often tolerating inhos-pitable ionic and osmotic conditions, environments with very high or low pH, extremes in salinity, and even periods of dehydration. For instance, cichlids live in the alkaline waters of Lake Magadi (pH 10) and tambaqui thrive in...
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- Fall '10
- Metamorphoses, Di Re