 Some tissues such as muscles can be made transparent , provided either they are very thin or organised as regular layers or fibrils that are small compared to the wavelength of visible light . A familiar example is the transparency of the lens of the vertebrate eye , which is made of the protein crystallin , and the vertebrate cornea which is made of the protein collagen . Other structures cannot be made transparent , notably the retinas or equivalent light @-@ absorbing structures of eyes — they must absorb light to be able to function . The camera @-@ type eye of vertebrates and cephalopods must be completely opaque . Finally , some structures are visible for a reason , such as to lure prey . For example , the nematocysts ( stinging cells ) of the transparent siphonophore Agalma okenii resemble small copepods . Examples of transparent marine animals include a wide variety of larvae , including coelenterates , siphonophores , salps ( floating tunicates ) , gastropod molluscs , polychaete worms , many shrimplike crustaceans , and fish ; whereas the adults of most of these are opaque and pigmented , resembling the seabed or shores where they live . Adult comb jellies and jellyfish obey the rule , often being mainly transparent . Cott suggests this follows the more general rule that animals resemble their background : in a transparent medium like seawater , that means actually being transparent . The small Amazon river fish Microphilypnus amazonicus and the shrimps it associates with , Pseudopalaemon gouldingi , are so transparent as to be " almost invisible " ; further , these species appear to select whether to be transparent or more conventionally mottled ( disruptively patterned ) according to the local background in the environment . 
