Breakthroughs and blind ends.
Evolution of a through gut with an oral and an anal opening conceivably marked a breakthrough in early animal history, as basal animals have a blind ending gut with only one opening. How this occurred, and whether a through gut evolved only once or several times is a matter of debate. Perhaps the different modes by which guts develop in animal embryos that are used for the classification of animals can shed some light on this question.
Maderspacher
Florian
F
D000818:Animals; D005075:Biological Evolution; D041981:Gastrointestinal Tract / Q000033:anatomy & histology / Q000502:physiology; D009024:Morphogenesis