The digestive tract of Bilateria is a tube with a mouth at one end and an anus at the other end. Radiata, that include the phylum Cnidaria, have a blind-sac form of digestive tract with only one opening. It has therefore been commonly believed that the evolution of the body plan from Radiata to Bilateria included the change of the digestive tract from a blind sac to a tube. In this study, we report that there is a very narrow opening at the aboral end of hydra termed the aboral pore. This confirms a classical finding by Kanajew (Zool Anz, 76:37–44, 1928), but we confirmed it in both asexually reproduced and sexually reproduced polyps, demonstrating that the aboral pore represents innate morphology. We also find that the opening coincides with the site where synthesis of an extracellular matrix-degrading enzyme, hydra matrix metalloprotease, is elevated suggesting that the pore is maintained by extracellular matrix degradation. Finally, we find that there is material transfer through the opening in both inward and outward directions. From these observations, we conclude that the digestive tract and the body plan of hydra is not a blind sac as formerly believed but is a tube with a tapered end.
Anterior posterior axis Oral aboral axis Blind sac body plan Gastraea theory
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The authors thank Dr. M. Martindale at University of Hawaii for numerous suggestions, discussions, and information. This work was supported in part by a grant from Japanese ministry of education for HS (no. 60170191).
Movie 1 Ingestion of India ink solution into the gastrovascular cavity through the aboral pore in floating polyps (MOV 389 kb)
Movie 2 Ejection of digested materials through the aboral pore (MOV 301 kb)
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