Classis Ophiuroidea Gray, 1840
Ophiuroidea are closely related to
starfish and are also regular radial animals, whose body consists of a
flat central disk and five rays. Unlike starfish, the
body is very small, disk-like and distinctly separated from the very
thin and long rays called arms. The mouth is in the middle of the lower
(abdominal) side and is pentagonal on account of the five jaws with dental
pipillae projecting into its opening. The dorsal and abdominal sides
are covered with small scales, and there are large plates—radial shields--
at the base of the rays. On the abdominal side there are no pronounced
ambulacral grooves, and ambulacral legs protrude through special openings
(ambulacral pores) in skeletal plates.
Unlike starfish, Ophiuroidea arms are articulated, and their skeleton consists of a longitudinal row of concatenated vertebrae brought into motion by intervertebra muscles. Each segment or vertebra consists of dorsal, abdominal and a pair of side plates, the last of which is normally armed with needles. Thanks to their muscles, Ophiuroidea arms are very mobile, and they can literally walk on their arms, lifting their central disk above the substrate. They have ambulacral legs on their arms, but, unlike starfish, hardly use them to move. They use them to breathe and feed. When they feed, Ophiuroidea lift up their arms almost vertically above the central disk. In some Ophiuroidea, their arms dichotomically branch to resemble a continuous carpet of feelers spread over the floor surface in accumulations of feeding Ophiuroidea. In this dense forest, small animals or suspended food particles either get stuck or seized. With respect to eating habits, such Ophiuroidea are mucous-ciliate filterers. There are also detritus- and cadaver-eaters among Ophiuroidea.
Unlike in starfish, in Ophiuroidea the madrepore plate of the ambulacral system is located on the lower (abdominal) side of the body, and the anal opening is absent altogether.
Ophiuroidea develop either directly or with plankton larval stage. Larval development is with dipleurula and two pluteus stages, which in Ophiuroidea are called ophiopluteuses. In direct development, Ophiuroidea are found to be partially viviparous. The point is that they have ten breathing sacks or bursas on the abdominal side near the stomach. The eggs of such Ophiuroidea develop in bursas, and in some forms the embryos even attach to the bursa wall.
In recent fauna, there are almost two thousand Ophiuroidea species, most of them inhabiting tropical waters. According to our data, at least twenty species live in Southern Primorye. Let us examine only the most interesting and frequently occurring representatives of this class.