One important concept is that typically used reinforcing (post-tensioning tendons and shrinkage-compensating concrete reinforcement are the exceptions) will not prevent the concrete from cracking. The reason for this is that the reinforcing cannot begin to start resisting significant tension until the concrete cracks.
In many bivalves and snails, and in cephalopods, part of the mantle is modified into a siphon, which can be used to pump water through the mantle cavity for respiration, feeding and/or jet propulsion. Freshwater and marine mollusks have gills (called ctenidia) for respiration, located in the mantle cavity.he mantle also secretes the shell, in those mollusks that have one. In many gastropods and cephalopods, the mantle is brightly colored and important for communication. In giant clams, the outer mantle tissue is colonized by symbiotic algae that provide their host with food energy in exchange for shelter.
Gills are evaginations of the body surface. Some open directly to the environment; others, as in fishes, are enclosed in a cavity. In contrast, lungs represent invaginations of the body surface. Many invertebrates use gills as a major means of gas exchange; a few, such as the pulmonate land snail, use lungs. Almost any thin-walled extension of the body surface that comes in contact with the environmental medium and across which gas exchange occurs can be viewed as a gill.