Ultrastructure of the egg shell of the acanthocephalan Neoechinorhynchus crassus.

In the acanthocephalan Neoechinorhynchus crassus five embryonic envelopes have been identified, including four, which are similar to those in previously studied acanthocephalans and a supplementary one, located between the outer and the second envelopes, which is characteristic of only a few Eoacanthocephala. The terminology of the envelopes in relation to their origin and possible function is discussed.