Sigillina Savigny , 1816

Kott, Patricia, 2007, Taxonomic affinities of three stalked colonial species of the Ascidiacea (Tunicata) from the central coast of New South Wales and indications of a trans-Tasman connection, Journal of Natural History 41 (9 - 12), pp. 633-645 : 642-643

Genus Sigillina Savigny, 1816

Type species: Sigillina australis Savigny, 1816

Caullery (1909) treated the genus Sigillina as a genus of the Distomidae halfway between the Polyclinidae and other Distomidae , having heart and gonads in the abdomen ( Distomidae ) rather than in the posterior abdomen ( Polyclinidae ), but with a vascular stolon (a possible homologue of the polyclinid posterior abdomen) as the site of replication. Kott (1990), accepting Caullery’s arguments supporting the affinity of the genus with distomid genera (which then were thought to include Sycozoa and Distaplia ), assigned Sigillina to the Holozoidae on the basis of its mode of replication by horizontal division of a vegetative stolon involving an epicardial (endodermal) sac (as in other holozoids), a short gut loop and fertilisation and incubation of embryos in a brood pouch formed from a loop of the oviduct separated from the zooid at the top of the oesophagus by a short, narrow constriction. However, there is no evidence that fertilisation is in the brood pouch in Sigillina and in the oviduct in Pseudodistoma . Similar developmental sequences at the top of the oviduct (see Kott 1990, 1992) occur in Sigillina fantasiana ( Kott, 1957) and Pseudodistoma aureum ( Brewin, 1957) . Also although the adhesive organs of Sigillina with platforms of branched clumps of columnar cells surrounded by a shallow epidermal cup differed from the large, balloon-like adhesive organs of some Pseudodistoma spp. This was not necessarily evidence of a holozoid relationship for Sigillina .

Despite these arguments, Kott (1998, 2005) subsequently observed that Sigillina and Pseudodistoma have more characters in common than Sigillina has with holozoid genera. Both Sigillina and Pseudodistoma have separately opening apertures and lack the cloacal systems (usually found in the Holozoidae ); three rows of stigmata (fewer than in any genus of the Holozoidae ) and a vertical gut-loop of moderate length. They both have large bands of longitudinal muscle on each side of the vascular stolon (like those on the posteriorabdomen of Polyclinidae and related families, while the vegetative stolons of holozoid genera do not have muscles in the body wall). In Sigillina , a conspicuous vascular stolon posterior to the abdomen with strong bands of longitudinal muscles on each side contains an extension of the epicardium. It differs from the posterior abdomen of Pseudodistoma only in lacking the gonads and heart (present in the abdomen in Sigillina rather than in the posterior abdomen as they are in Pseudodistoma ). The limited number of transverse muscles in the thoracic body wall of both these genera supports their relationship with one another rather than with any genus of the Polyclinidae . Further, larvae of Sigillina and Pseudodistoma both have large antero-median larval adhesive organs surrounded at their base by ampullae or stalked vesicles, while larvae of holozoid genera are distinct in having triradially arranged adhesive organs with cones of adhesive cells.

It is possible that the presence of gonads in the posterior abdomen in Pseudodistomidae is not such a compelling difference from Sigillina as was implied by their assignation to different families. As argued in the present work, they appear to have a close phylogenetic relationship justifying the inclusion of Sigillina with Pseudodistoma in the Pseudodistomidae . The presence of gonads in the posterior abdomen of Pseudodistoma , suggesting an affinity with the Euherdmaniinae in the family Polyclinidae , undoubtedly obscured the possible relationship of this genus with Sigillina (see Kott 1992, 2006).