Holothurians or sea cucumbers
classis Holuthuroidea Zittel, 1883

    Like sea urchins, holothurians have no arms, which are included in a body that had lost its regular five-ray radial shape. The holothurian body  is normally bilaterally-symmetrical, elongated along the mouth-anus axis; is worm-like, cylindrical, less often spheroidal. Holothurians lie, as it were, on their side, the side carrying three rows of ambulacral lets (trivium) being the abdominal one, and the side with two rows of ambulacral lets (bivium) the dorsal one. The rows of legs in holothurians are called ambulacras. The calcareous skeleton is reduced and represented by isolated microscopic bonds, but in rare cases may retain a scaly cover on the back. At the same time, holothurians have a thick and elastic body wall with well developed annular and longitudinal muscle clusters, the last of which are represented by five wide strips. The mouth is located on the anterior body end and normally surrounded by a corona of 8-30 feelers of diverse shape: fingerlike, branched and corymbose. The feelers are used to feed with detritus or to filter food from near-floor waters. The madrepore plate of the ambulacral system in holothurians lies freely in the body cavity.
   Holothurians breathe with water lungs—sacks that branch from a cloaca representing a widening of the posterior section of the digestive system directly in front of the anus. The water fills these lugs and is then forcibly ejected outside. Sometimes, when stimulated or threatened, holothurians eject (eviscerate) the lungs per se along with their posterior intestine. This is often accompanied by ejection of toxin-containing Cuvier tubes.
    Holothurians move with the aid of their ambulacral legs, which are situated not only in the ambulacres, but also often scattered over the entire body surface. Abdominal legs, used more often to move, are normally supplied with suckers, while dorsal legs are often transformed into soft papillae. In deep-water holothurians, ambulacral legs may be highly elongated to be used as stilts. Certain holothurians totally lack ambulacral legs and move by peristaltic contractions of the body wall muscles, pushing away from the soil with their protruding calcareous bones. Holothurians are the only groups of Echinodermata we examined (except larvae) that also have plankton, i.e. freely swimming, representatives.
    Holothurians are frequently observed to care for their embryos, which develop in their body cavity, in rare cases in the ovary. In indirect development, there are freely swimming larval stages, viz. vitellaria, auricularia, doliolaria and pentactulas.
    Recent fauna is represented by 1,150 species, According to our data; Southern Primorye is inhabited by twenty-one holothurian species. Examined below are the most common varieties.