Holothuroidea Have Spongy Bodies Homologous to Spongy Bodies of Echinoidea and Tiedemann’s Bodies of Asteroidea

Organs that can be considered as homologous to the spongy bodies of sea urchins and Tiedemann's bodies of sea stars were found in the holothuroid Chiridota laevis (O. Fabricius, 1780). The C. laevis spongy bodies occur in interradii and are formed by outgrowths of coelomic canals, which connect the water ring with the coelomic cavities of tentacles. Haemocoelic lacunae are adjacent to the coelomic outgrowths. The spongy bodies of sea urchins and Tiedemann's bodies of sea stars are also in the interradii and indicate the position of reduced tentacles. The spongy bodies were assumed to function as excretory organs (additional kidneys) in echinoderms. Because the water vascular system does not open outwards in most holothuroids, the holothuroid spongy bodies are much smaller than in other echinoderms and can be considered as rudimentary organs.