What do you expect, when it all started ca 380 Mio year ago. D.P. Knight Biosciences, University College, Winchester Coverings of eggs laid in water must protect the embryo from mechanical injury, predation, osmotic shock, ultraviolet light, toxins, infections and fouling while providing for efficient gas exchange, retention of nutrients, removal of nitrogenous waste, attachment or dispersal, and for appropriately-timed hatching of the embryo. In addition, shells of eggs laid in air often need to give more support and must reduce the rate of desiccation to retain sufficient water for continued life or to enable an anhydrobiotic state to develop. Egg shells of parasites may have to survive digestion in the gut, and may require specialised means of passing through tissue barriers to reach the outside world. Liquid crystalline assembly provides mechanically favourable molecular architecture.
Pores maintain sufficient gas exchange for respiration while limiting water loss.
Calcium salts and crystalline proteins provide stiffness. Hatching is frequently facilitated by a spine on the embryo to increase local stress concentration on the inner surface of the egg shell and by dissolution of general shell components or a specialised hatching seam. R.J.A. Atkinson University Marine Biological Station, Millport Marine sediments are often extensively burrowed. Only since the advent of in situ observation techniques has the extent of this become apparent. The megafaunal architects include crustaceans, fish, echiurans and bivalves.