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

232 ECHINODERMATATypically holothuroids are deposit feeders. A group of anteriortube feet forms long buccal tentacles that are held out as a netor gather food from the sand. Locomotion is slow, by worm-likewriggling of the muscular body wall, or there may be locomotortube feet in ambulacral grooves on the underside of the body.When pursued by predators, holothuroids may immobilise themby extruding a mass of very sticky blind-ending tubules, or maybreak off the internal organs and leave them in the path of thepredator: this is achieved by complete slackening of the groundsubstance of the mutable connective tissue. The rest of the holothuroidescapes, and regenerates its viscera.17.3.7 ConcentricycloideaThe concentricycloids (sea daisies) were discovered only in 1986 indeep seas off New Zealand; a second species of Xyloplax has nowbeen found in the Caribbean. Concentricycloidea are at presentconsidered to constitute a separate class closely related toAsteroidea: they may even be placed within that class. Superficiallythey resemble medusae (Figure 17.7c) but there is a water vascularsystem with two connected concentric rings, one of which bearstube feet. The spermatozoa are very unusual and, uniquely in thephylum, fertilisation may be internal. Feeding methods are unknown(one of the species has no gut) but absorption of dissolved organicmaterials is probably important.17.4 Dothelarvaeilluminateechinodermevolution?Free-living larvae, in most phyla, are simpler than the adults.This does not mean that they represent ancestral forms; they areevolutionary novelties, adapted to their own way of life. Referenceto ancestral forms is especially tempting in those echinodermswhich have bilaterally symmetrical larvae but bilateral symmetryis to be expected in motile animals. Motile planktonic larvae arefound in most echinoderms, and are believed to be primitive tothe phylum. Some of the species that instead develop directly fromyolky eggs are found to contain vestigial larval structures, whichsupports the idea that direct development is a secondary specialisation.Planktonic larvae are an advantage to sessile or slow-movinganimals living on the sea bottom, because they can exploit a differentfood source and can disperse the species. They also allow greaterfecundity, since a large number of small eggs without food reservescan be produced.Figure 17.8 indicates the larvae characteristic of the differentclasses. The prevalence of convergent evolution is instantly apparent.Larvae are above all adapted to their own way of life. That bothophiuroids and echinoids have a ‘pluteus’ larva with the ciliary bandextended into arms by eight skeletal rods does not mean that brittle