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

WHATARE THE CLOSEST RELATIONS OF ARTHROPODS?165information to the brain. The ommatidia are separated by pigment,movable so that more or less light can be admitted.A compound eye suggests a mosaic, with each ommatidiumsensitive to a particular spot on the whole visual field. Mostimportantly, however, each ommatidium receives some of the lightfalling on its neighbours, so that the visual fields of neighbouringommatidia overlap. Such a structure is extremely sensitive tomovement, over a large field of vision, and this is the great advantageof a compound eye. It also provides magnification, and owing to theshort light path may readily detect short wave lengths, for exampleultraviolet light. Compared to the vertebrate (or cephalopod) eye ithas, however, poor resolution and image formation.The above generalised discussion indicates that most of thecharacteristic features of arthropods relate to their exoskeleton, andbegins to explain the foundation for their diversity, both betweenand within the main groups.12.5 What are the closest relations of arthropods?The present-day arthropods are Crustacea, Chelicerata, Myriapodaand Insecta, and these will be introduced in the following chapters.Fossils, in particular one large fossil group, the trilobites, are theoutstanding omission from this account (see Chapter 2).Two small groups once included as ‘arthropod-like’, or ‘protoarthropods’,are Pentastomida, now recognised as modified parasiticcrustaceans (Chapter 13) and Pycnogonida, now included as chelicerates(Chapter 14).Molecular and morphological evidence combines in identifyingthe two groups of non-arthropods closest to the arthropod ancestors,the Tardigrada and Onychophora.12.5.1 TardigradaDespite some similarities such as the moulted cuticle, the tardigradesare not arthropods but constitute a separate phylum. These attractivelittle animals (Figure 12.8a) are the koalas of the invertebrate world.Some 700 species are known, only 0.1 1.2 mm long, living in waterfilms on mosses or other plants in damp places or between particlesin freshwater (less commonly marine) sediments. They feed mainly onbacteria. They have seven to eight segments bearing short lobe-likelegs consisting mainly of claws, and a ventral nerve cord withsegmental ganglia. Muscles radiate from the leg base and there is nocircular muscle. These small animals have no true coelomic cavities,no blood vessels and no segmental excretory organs; structuresresembling Malpighian tubules regulate the ion content of the bodyfluid and open into the gut. In dry conditions tardigrades survive byforming ‘tuns’, barrel-shaped resistant resting stages with greatlyreduced metabolism; these may also be dispersal agents.