An Introduction to the Invertebrates, Second Edition - tiera.ru

228 ECHINODERMATAring but in the radial nerve cord: patterns of activity in any onearm can be conducted round the ring and direct the stepping oftube feet in the animal as a whole. In this way a starfish can makea temporary ‘brain’ without having a permanently defined brainstructure.17.3.4 OphiuroideaThe ophiuroids (brittle stars) constitute another large class (2000species) of rather smaller starfish. They have very much more hardskeleton than asteroids and, although they are stellate in form, thearms are clearly marked off from the disc (Figure 17.4a). Most of thearm consists of central muscle surrounded by ossicles (Figure 17.4b,c)which are able to move on each other and give the arms greatflexibility (hence the name ‘serpent stars’). These ossicles are oftencalled ‘vertebrae’, and indeed no other invertebrates have suchvertebrate-like structures. Most ophiuroids resemble asteroids inbeing active carnivores, but their methods of feeding and locomotionare very different. Food is seized by the mobile arms, assisted bymucus secreted by tube feet that have no ampullae and play onlya minor role. Ophiuroids walk by two arms on each side sweepingback from the leading arm; this ‘rows’ the animal along in a seriesof jerks, with the spines providing traction against the ground.Swimming is achieved by similar arm movements. Coordination ofthese relatively rapid movements is more highly developed thanFig.17.4 The structure ofophiuroids: (a) dorsal view ofan ophiuroid (brittle star);(b) transverse section of armof Ophiothrix;showingextensiveossicle; (c) vertical sectionof ophiuroid.