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Lattice organs: a review
Thecostracan lattice organs were first described by Elfimov (1986). Their status as synapomorphies for the Cirripedia and the Ascothoracida (in the latter group they were initially dubbed cardiac organs, but Jensen et al. (1994a) later identified them as lattice organs on grounds of homology) was firmly established by Itô & Grygier (1990) and Jensen et al. (1994a). Lattice organs have been found in the cyprid larvae of thoracican, acrothoracican, and rhizocephalan Cirripedia as well as in larval stages of Ascothoracida (Jensen et al. 1994a) and recently also in Y-larval Facetotecta (Høeg & Kolbasov 2002). The extant crustacean parasite Tantulocarida as well as the Cambrian Bredocaris have been brought forward as potential sister groups of the Thecostraca (Walossek et al. 1996; Walossek & Müller 1998), but both groups definitely lack lattice organs. However, within the Thecostraca the number, position and anatomy of lattice organs constitute systematically valuable characters (Høeg & Kolbasov 2002).
In thecostracans, the lattice organ complex normally forms characteristic constellations of two clusters containing all together five pairs of lattice organs. Typically, the anterior cluster contains two pairs while the posterior usually includes three pairs. However, some species have a reduced number of lattice organs while a varying number of ‘extra’ pore fields have been registered in the carapace of other species. Such ‘extra’ fields resemble lattice organs but display a more irregular layout and lack the terminal pores found in most ‘real’ lattice organs (Jensen et al. 1994a).
The outline of the lattice organs varies from very elongate to roundish; and from a deep depression that contains a central raised smooth keel, to simple flat pore fields in level with the surrounding carapace cuticle. Intermediates of these principal types may have less pronounced keels with a reticulated surface comparable to the pore fields. One so-called terminal pore is often associated with each lattice organ. A rim of cuticle sometimes delimits the lattice organs (Jensen et al. 1994a; Jensen et al. 1994b). Often they are minute structures of 5-10 µm length and 1-2 µm width but dimensions vary with species and some rather ‘gigantic’ lattice organs approach 50 µm length or 10 µm width (including the surrounding rim) in some of the larger lepadid species (Jensen et al. 1994a).
New evidence demonstrates that the lattice organs are derived setae (Rybakov et al. in press) presumably with a chemosensory function (Høeg et al. 1998).Dendrites from sensory cells project into the cuticle and terminate in proximity to the pores. The larger terminal pore seems the most likely pathway for molecules to reach the sensory elements in Facetotecta and Ascothoracida. In Thoracica and Rhizocephala, the many small pores of the pore field may facilitate diffusion towards the sensory elements. These pores reside in the exocuticle with the thin epicuticle coating the walls and the bottom of the pores. In Acrothoracica, minute pores penetrate the epicuticle but not the underlying exocuticle. The possession of numerous pores may be autapomorphic for the cirripedian lattice organs (Høeg et al. 1998; Høeg & Kolbasov 2002).