The Mouth Cone and Mouth Ring of Echiniscus viridissimus Peterfi, 1956 (Heterotardigrada) with Comparisons to Corresponding Structures in Other Tardigrades

Abstract

The mouth cone and mouth ring of the tardigrade Echiniscus viridissimus are described and their primary homology is assessed by comparing them to structures considered to be homologous in other heterotardigrades, eutardigrades, and ecdysozoans. In E. viridissimus the mouth cone is divided into anterior and posterior regions that differ in the ultrastructure of their cuticle. The mouth ring of the buccopharyngeal apparatus lies just anterior to the buccal tube. It is connected to it by a narrow flange or ridge of cuticle that allows the rostral end of the mouth tube to telescope part way into the mouth ring. Sensilla innervating the mouth cone and mouth ring in E. viridissimus correspond to sensilla of the circumoral and suboral sensory regions described for eutardigrades. The sensory fields, of both the mouth cone and mouth ring, exhibit two planes of mirror image or biradial symmetry. Although the relative dimensions of mouth ring differ markedly between E. viridissimus and eutardigrades, homologous areas can be identified easily. For example, the supporting rods of the mouth ring in E. viridissimus appear to be homologous to the supporting plates in the lamellae of eutardigrades. The widespread occurrence of the mouth cone and mouth ring in hetero- and eutardigrades suggests that these structures were present in the last common ancestor of all tardigrades. Expression of these characters in other ecdysozoans is more problematic. Tentatively the mouth cone and mouth ring are considered to be homologous to the circumoral plates of onychophorans and the ring of radiating plates found in the ‘Peytoia’ apparatus of Cambrian arthropods. These structures are not thought to be homologous to the introverts of scalidophorans or the circumoral structures of nematodes or nematomorphs, and are considered to be absent in these taxa. Thus, the mouth cone and mouth ring are potentially key synapomorphies for Panarthropoda.