Synapomorphies of Annelida
The monophyly of Annelida is not well supported and only two morphological features are worthy of discussion; segmentation and chaetae. Nuchal organs represent another possible apomorphy and are discussed in the section on sensory structures (see plesiomorphies and other features).
1. Metamerism (segmentation)
Annelids have three body regions (Fig. 2). The majority of the body is comprised of repeated units called segments. The original French use of the name Annélides (Lamarck, 1802) comes from the Latin word ‘anellus’, meaning a little ring, in reference to the presence of the ring-like segments. Each segment is, in principle, limited by septa dividing it from neighbouring segments, and has a fluid-filled cavity within referred to as a coelom. Structures such as the excretory, locomotory and respiratory organs are generally repeated in each segment. Segments are formed sequentially in annelids and are established during development from growth zones located at the posterior end of the body; so the youngest segment in the body of an annelid is always the most posterior. The only parts of the annelid body that are not segmental are the head and a terminal post-segmental region called the pygidium. The head is comprised of two units, the prostomium and the peristomium. The postsegmental pygidium includes the zone from which new segments are proliferated during growth. The proposed homology of segmentation seen in annelids with that seen in Arthropoda has been used to unite the two as Articulata, a grouping that dates back to Cuvier (1817). The homology of this segmentation has been questioned recently, with arthropods now viewed by many as closer to taxa such as Nematoda (Aguinaldo et al., 1997). This suggests that the form of segmentation seen in annelids may in fact represent an apomorphy. With regards to the supposedly unsegmented Echiura, their reinstatement within Annelida (see McHugh 1997) suggests that their apparently unsegmented body in fact represents a series of fused segments (see Hessling and Westheide 2002).
A distinctive feature of annelids are structures called chaetae (Fig. 3). Chaetae (also called setae) are bundles of chitinous, thin-walled cylinders held together by sclerotinized protein. They are produced by a microvillar border of certain invaginated epidermal cells and so can be defined as cuticular structures that develop within epidermal follicles. Chaetae show a huge amount of variation, from long thin filaments (capillary chaetae) to stout multi-pronged hooks (Fig. 3). Apart from annelids, chaetae are found in Echiura and Brachiopoda. There is now good evidence (Hessling and Westheide, 2002; McHugh, 1997) that the former group falls within Annelida. The position of Brachiopoda is controversial (Lüter, 2000b; Lüter and Bartolomaeus, 1997; Stechmann and Schlegel, 1999) and the homology of their chaetae with those of annelids is unresolved (Lüter, 2000a). There is a distinct possibility therefore that chaetae represent an apomorphy for Annelida.
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