Evolutionary trends in sperm structure.

In the marine invertebrates which practise external fertilization, the spermatozoon is a flagellate cell, possessing an acrosomal complex made up of acrosome and perforatorium. In the animal phyla which adapted to land after having achieved internal fertilization the spermatozoon has acquired an elongated shape and develops accessory cytoskeletal organelles in the tail. In the highest forms, the flagellum seems to have become too long for perfect motility; in such a condition, aberrant axonemal patterns can appear and aflagellate spermatozoa can also be found. At the apex of the terrestrial phyla, alternative motile apparatuses have been achieved and also the acrosomal complex varies, according to the different structures of egg envelopes.