CHAPTER 15 



SKELETONS, SHELTERS AND SPECIAL DEFENCES 



For they had heard that in certain parts of the ocean a kind of worm is 

 bred, which many times pierceth and eateth through the strongest oak 

 that is : and therefore that the manners and the rest to be employed in 

 this voyage might be free and safe from this danger, they cover a 

 piece of keel of the ship with thin sheets of lead . . . 



Hakluyt: Chancellor's North-Easterly Voyage, 1553 



Previously we have noted how certain inorganic substances are concen- 

 trated in skeletal structures (Chapter 2). In the following pages the skele- 

 tons of marine animals are considered in more detail. Skeletons are 

 internal in some animals, but more often appear as external coverings, 

 shells, etc. Instead of forming hard exoskeletons, many animals secrete 

 hard tubes of calcareous or organic matter, and others utilize external 

 secretions for binding together foreign materials or for lining burrows. 

 Empty, or even inhabited, shells and burrows are invaded by soft-bodied 

 species seeking protection. This leads us to a consideration of burrowing 

 and boring species, and the mechanisms which they employ. Finally, we 

 shall examine special protective devices, such as poison glands, which are 

 frequently associated with piercing spines and lancets, and this will afford 

 an opportunity for dealing with poisons and poisonous secretions among 

 marine animals. This assemblage of topics may appear somewhat hetero- 

 geneous, but the structures under discussion are linked by a common pro- 

 tective and defensive role in the economy of the organism. 



SKELETONS 



Animals are provided with hard skeletons for support, protection and 

 defence. Endoskeletons protect soft and delicate tissues, e.g. the brain of 

 cephalopods, cyclostomes and fishes. They give internal support to the 

 body wall and appendages, e.g. in cephalopods, fish ; and form rigid sup- 

 ports and lever systems for the operation of muscles in certain molluscs, in 

 crustaceans and in chordates. The exoskeletons of different animals have 

 similar protective and functional roles and also provide barriers to the 

 diffusion of water and solutes. 



Endoskeletons 



Endoskeletal structures lie either just beneath the surface or at deeper 

 levels in the body. Although particularly characteristic of vertebrates, we 

 find endoskeletons present in several invertebrate phyla, especially sponges 



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