Hejnol, Andreas, and Mark Q. Martindale. "The mouth, the anus, and the blastopore—open questions about questionable openings." Animal Evolution: Genomes, Fossils, and Trees. : Oxford University Press, 2009-08-01. Oxford Scholarship Online. 2009-09-01. Date Accessed 20 May. 2013 <http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780199549429.001.0001/acprof-9780199549429-chapter-4>.
Hejnol, A., & Martindale, M.(2009-08-01). The mouth, the anus, and the blastopore—open questions about questionable openings. In Animal Evolution: Genomes, Fossils, and Trees. : Oxford University Press. Retrieved 20 May. 2013, from http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780199549429.001.0001/acprof-9780199549429-chapter-4.
Hejnol, Andreas, and Mark Q. Martindale. "The mouth, the anus, and the blastopore—open questions about questionable openings." In Animal Evolution: Genomes, Fossils, and Trees. : Oxford University Press, 2009-08-01. http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780199549429.001.0001/acprof-9780199549429-chapter-4.
Hejnol A, Martindale MQ. The mouth, the anus, and the blastopore—open questions about questionable openings. In: Telford MJ, Littlewood D. Animal Evolution: Genomes, Fossils, and Trees. Oxford University Press; 2009. http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780199549429.001.0001/acprof-9780199549429-chapter-4. Accessed May 20, 2013.