Advantages and Disadvantages of an Exoskeleton

More than 80 percent of the points of attachment for muscles, provid- 

Despite all their good points, exoskeletons have some drawbacks. They are heavy, so the only animals that have been successful with them over time are those that have remained small. In addition, an animal must molt, or shed, its exoskeleton to grow. During and immediately after a molt, an animal is unprotected and vulnerable to predators. 

In most arthropods, sexes are separate. Mating occurs at prescribed times of the year, and courtship rituals are often complex. In many species the male deposits sperm in the female s body. The sperm are held there until eggs mature, then as each egg leaves the ovary, sperm are released and fertilization occurs. Resulting zygotes mature into larvae that swim in the plankton for a short period of time before settling down on the seafloor to mature. 

Barnacles are sessile crustaceans that live within protective plates. Positioned so that their heads are down and their legs are sticking out into the water, barnacles spend their lives attached to wood, rock, or some other solid surface. At low tide, when they are exposed to air, barnacles close their plates to prevent drying. These plates have a small opening that allows moist air to circulate over their gills, providing oxygen. 

Unlike most of their close relatives, barnacles are hermaphrodites, but individuals usually cross-fertilize. One barnacle can transfer sperm to its neighbor by way of a long, extensible tube. Fertilized eggs develop into zygotes that later form swimming larvae. Eventually, the larvae settle on a substrate, exude a brown glue that anchors them in place, and metamorphose into the adult form. 

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