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

WHICH ANIMALS HAVE LOPHOPHORES?215only phylum to be entirely colonial, with zooids about 0.5 mm long,and the largest class is marine, but there is one class that is entirelyfreshwater. Another is known mainly as fossils, very abundantsince the late Cambrian. Bryozoa (‘moss animals’) have also beencalled ‘Polyzoa’ (because there are many individuals in a colony)or ‘Ectoprocta’ (because, as with all true lophophores, the anus isoutside the tentacular ring).Bryozoa are sessile colonies of miniaturised animals. Microscopicexamination shows that each inhabits a separate little ‘box’ of cuticlewithin the colony, which at once distinguishes them from hydroidcnidarians, and their high speed of withdrawal into their boxes isa further distinction. The cuticle is made of chitin and protein,sometimes also calcium carbonate. The zooid body typically hasa stationary trunk and an eversible lophophore, extended for feedingwhen muscular contraction increases the fluid pressure in thecoelom and a special opening appears. There is a simple nerveganglion giving off nerves, a U-shaped gut, and (in such smallanimals) no excretory or blood systems. The colonies grow bybudding, or the zooids may contract down with regression of thegut and lophophore, leaving remnants as ‘brown bodies’ when mostof the tissue regenerates. The zooids are hermaphrodites, the gametesbeing shed into the coelom and passing out through pores at thetentacular base. The fertilised eggs are usually brooded, and hatchas free-swimming larvae.Bryozoa are diverse in form. Encrusting species grow as flatsheets covering seaweed, upright species grow vertically up fromthe substratum as tree-like tufts or ‘bushes’. Division of labourbetween the members of a bryozoan colony commonly occurs. Inaddition to feeding zooids (described above) there may be ‘bird-like’individuals with jaws serving for defence and ‘whip-like’ ones thatclear away waste and deter settlers (Figure 16.1c).The metabolism of Bryozoa shows great diversity: for example,in the Antarctic there are some species with the fastest metabolicrates known in polar animals and other species with the slowest.The great success of the phylum over evolutionary time is thoughtto be associated with such metabolic flexibility.16.2.3 BrachiopodaThere are about 13 000 species, of which only 350 are living today.The shell is less than 100 mm long or wide. All are marine.Brachiopods are solitary sessile animals enclosed by the dorsal andventral valves of a shell (Figure 16.1d). The two halves of the shellare unequal and the ventral half bears a stalk that anchors theanimal to the substratum. Resemblance to the bivalve molluscs,where the two halves of the shell are lateral and equal, is onlysuperficial. The main body of the brachiopod consists of a coelomsurrounding the gut, excretory metanephridia and either testesor ovaries, as the sexes are separate. There is an open blood system