Sperm come in a wide variety of shapes but in general tend to be elongated, to resemble simple filaments. The sperm of crayfish are star-shaped, while some have undulating membranes, explosive capsules, etc. It has been said that sperm never exceed 50 microns (1 micron=1 thousandth of a millimetre) in diameter, but there are a number of species in which the sperm attain remarkable lengths. In amphibia of the family Discoglossidae sperm can sometimes be as much as 2.mm in length, and in the aquatic Notonecta the staggering length of 12 mm. is attained! Perhaps even more remarkable is the fact that in the ostracods, minute crustaceans, the sperm are six times as long as the entire body of the animal! When they are generated they are curled up in compact form.