The synthesis of literature and personal observations permitted to show the main morphological physiological and genetic characteristics with relation to the mode of life of abyssal echinoderms. The large ventral sole is common among holothuroids and asteroids which creep on the bottom. The development of large podia and their ability to swim are particular to the abyssal family Elasipoda (Holothurioidea). The paper gives examples of opposite skeleton constitution either heavily or lightly calcified. The little knowledge about the biology of these animals shows the absence of a strict diet and of a particular mode of reproduction. [NOT CONTROLLED OCR]