INTRODUCTION TO THE SPIRALIA

Spiralia, a group was first recognized by Halanych et al. (1995), who called it the Lophotrochozoa, includes two major clades (see Figure 1) of protostome animals: Trochozoa (Roule 1891), and Platyzoa (Cavalier-Smith 1998). Though quite diverse, the unifying synapomorphy for the Spiralia according to Giribet (2002 and 2008) and Edgecombe et al. (2011) is spiral cleavage (see Figure 2). This cleavage pattern is highly stylized and unlikely to have evolved more than once. Nevertheless, taxa in some lines have reverted to the simpler radial cleavage (Edgecombe et al. (2011).

THE TROCHOZOA

Trochozoa has a set of characters, chiefly trochophore larvae and lophophore feeding tentacles, that unite members of the clade, including the nested clades of Polyzoa (Thompson 1830) and Brachiozoa (Cavalier-Smith 1998). We use formal term, Eutrochozoa (Eernisse et al. 1992) in a narrow sense to refer to the monophyletic group that includes Annelida and Mollusca. The trochophore larva (Figure 2) is a free-swimming plankton-feeding stage with a gut and a distinctive ring of cilia. Animals such as annelid worms and molluscan snails produce trochophore larvae in their developmental life histories; so, although the adults are quite different, the larval forms show the relationship. This, also, has been borne out by a body of molecular phylogenetic work (summarized by Edgecome et al. 2011). Lophophores are ciliated feeding tentacles (see Figure 3) that usually occur in rings associated with the mouth. Halanych et al. (1995) helped to resolve a long-standing debate about the placement of the lophophorates (lophophore-bearing taxa; see Figure 3) in the bilaterians. Were they deuterostomes (Nielsen, 2001 and Margulis & Schwartz 1998) or protostomes? Furthermore, Nielsen (2001), Halanych (2004), and Peterson and Eernisse (2001) considered the lophophorate condition to be ancestral in the clade of the Bilateria which would make it a plesiomorphic character within the Bilateria. Valentine (2004) reviewed molecular, fossil, structural, and developmental work that united the animals with trochophore larvae with those that had lophophores; however, he still could not find structural/developmental synapomorphies that defined them. Thus, Valentine (2004) presented the lophophorates and trochophores as separate clades within a group called the Lophotrochozoa. The structure of such a system breaks down because some taxa like Entoprocta have both lophophores and trochophore larvae. The bryozoan taxa, here referred to as the Polyzoa, include Ectoprocta and Entoprocta, phyla that have been associated (e.g. Helmkampf et al. 2008) and separated (e.g. Jang and Hwang 2009) based on various cladistic analyses, have been rejoined with the odd phylum, Cycliophora (e.g. Funch and Kristensen 1995; Passamaneck and Halanych 2006; Edgecombe et al. 2011). The other lophophore-bearing phyla, Brachiopoda and Phoronida have been joined in a clade called Brachiozoa (Cavalier-Smith 1998). This union, though questioned (Jang and Hwang 2009), has general support (Helmkampf et al. 2008; Edgecombe et al. 2011).

THE PLATYZOA

Giribet et al. (2000) identified a clade of pseudocoelomate taxa that included the Platyhelminthes, and thus they called the group the Platyzoa, a name coined by Cavalier-Smith (1998). The platyzoan clade included phyla that were pseudocoelomate, a persistent blastocoel also called a paracoelomic condition. Some were acoelomate, no body cavity, as adults (Platyhelminthes, Gnathostomozoa, and Gastrotricha; from Valentine 2004). Despite the absence of a true coelom (absence of a character is not a character), the only common feature that the taxa seemed to have in common was a ciliated epidermis. Valentine (2004) removed the Platyhelminthes (flatworms) from the clade and changed the name to Paracoelomata. More recent analyses merge the flatworms with other members of the Platyzoa, and so the name has returned to replace Paracoelomata (e.g. Helmkampf et al. 2008; Giribet 2008; Giribet et al. 2004; 2007; 2009; Dunn et al. 2008; Passamaneck and Halanych 2006; Edgecombe et al. 2011). Most of the phyla within the Platyzoa are very small and feed using a ciliary field around the mouth. In addition, they have distinctive jaw-like structures in the pharynx, which give the name to their clade, Gnathifera (Giribet et al. 2000), the jaw-bearers. Monophyly for this clade has been confirmed by Witek et al. (2009).

THE MESOZOANS, PROBLEMATIC PHYLA

The Rhombozoa and Orthonectida, collectively called Mesozoa, are parasites of molluscans. Though their developmental life histories are quite complex, the adult morphology is extraordinarily simple for a metazoan. Indeed, the name, Mesozoa implies that it is a group that occupies an intermediate stage between protozoa and metazoa. Developmental and molecular work suggest that they clearly are bilaterians and emerge from within the Spiralia (Furuya et al 1992). So far, the sister group has not been identified though some suspect that they are reduced Platyhelminthes (Stunkard 1954; Winnepenninckx et al. 1998). More recent work suggests that they are Trochozoa and related to Mollusca or Annelida (Furuya and Tsuneki 2003; Kobayashi et al. 1999).

We follow the synthesis of Edgecombe et al. (2011) in the development of the topology of Figure 1. The trochozoa has three major clades: Eutrochozoa, Brachiozoa, and Polyzoa. The platyzoa has two stem phyla, Gastrotricha and Platyhelminthes. The remaining phyla are in a clade called Gnathifera. Given this structure, we have provided a comprehensive taxonomic system to class. You may also follow the links below to pages on the respective phyla. Please consult The Major Clades of the Animal Kingdom for some views on the relationships of the protostome phyla with each other and with the other phyla of protostomes. Also, consult the Protostomata page for different cladistic treatments of the spiralian phyla.

• TROCHOZOA (Roule 1891)
  • EUTROCHOZOA (Eernisse et al. 1992)
    • ANNELIDA (Lamarck 1809)
    • MOLLUSCA (Linnaeus 1758)
    • NEMERTEA (Schultze 1851)
    • RHOMBOZOA (van Beneden 1876)
    • ORTHONECTIDA (Giard 1877)
  • POLYZOA (Thompson 1830)
    • ECTOPROCTA (Nitsche 1869)
    • ENTOPROCTA (Nitsche 1870)
    • CYCLIOPHORA (Funch and Kristensen 2005)
  • BRACHIOZOA (Helmkampf et al. 2008)
    • BRACHIOPODA (Dumeril 1806)
    • PHORONIDA (Hatschek 1888)
• PLATYZOA (Cavalier-Smith 1998)
  • GASTROTRICHA (Metschnikoff 1865)
  • PLATYHELMINTHES (Gegenbaur 1859)
  • GNATHIFERA (Kristensen and Funch 2000)
    • GNATHOSTOMULIDA (Ax 1956)
    • MICROGNATHOZOA (Kristensen and Funch 2000)
    • SYNDERMATA (Ahlrichs 1997) [=ROTIFERA]