These roll outward with a continual supply of teeth to replace those that are lost. Sharks' teeth are often found in the wounds of those attacked. Fossil teeth, found so abundantly in some areas, do not necessarily indicate the presence of multitudes of sharks but may rather be multitudes of teeth that were frequently lost and replaced. Sharks have strong digestive juices and a specialized type of intestine called a spiral valve that increases the secretory and absorptive surfaces considerably. An epithelial fold spirals the length of the small intestine.
Sharks exhibit all of the different kinds of reproduction.
In all sharks fertilization of the eggs occurs internally. During copulation only one clasper has been observed to be inserted into the female. Gestation varies from a few months to more than two years. The longest known gestation period of any vertebrate, 20-24 months, is that of the spiny dogfish. Unlike bony fish, which usually lay enormous numbers of tiny eggs that develop into minute young, sharks produce small numbers of large young.