New Characters Observed on Isolated Jaws and Basal Plates of the Family Gnathostomulidae (Gnathostomulida)

Summary

• 1. The jaws of the family Gnathostomulidae have four major parts (fig. 80) : articularium, involucrum, dentarium, and suspensorium.

• 2. The articularium is highly specialized and fully freed from functions other than articulation of the jaws and the prevention of twisting motions. It consists of a symphysis lamella, joined by a symphysis in vertical position.

• 3. The involucrum is a specialization in the higher families of the Scleroperalia and the Austrognathiidae. In Gnathostomulidae it is of medium length with a well-defined caudal end, surrounding an apertura caudalis. From there a much thinner tectum lateralis continues. It is formed by a dorsal extension of the lamella interna which bends-as lamella externa-laterally and then ventrally, leaving only a fissure-like opening medioventrally: the incisura ventralis.

• 4. The dentarium consists of a thin lamella interna, which is always thickened in three portions, forming the arcus dorsalis, medialis, and ventralis. These arcs form the bases of the teeth. The arcus medialis also bears the strong dens terminalis. The dentation is more complicated and minute than the light microscope can resolve. An incisura dorsalis is found in few cases, cutting into the lamella interna from the caudal end.

• 5. The suspensorium is specialized into two portions: an anchorage part at the more fixed end., and an apophysis part nearer the moving ends of the jaw system.

• a) The cuticularized parts of the cauda system are always paired, but can be symmetrically or asymmetrically developed. In the first case the cuticularized caudae are tube- or cushion-like; in the latter case they are tubeshaped again, but a cauda dorsalis and a cauda ventralis can be distinguished.

• b) The apophyses are wing-shaped only distally, proximally they are differentiated (fibularized) into two fibulae functioning as cuticularized sinews: the fibula medialis originates at the ventrocaudal end of the lamella interna, the fibula lateralis at the ventral margin of the lamella externa. Together they form the fenestra ventralis, varying in dimension.

• c) In addition a fibula radialis is developed, strengthening the apertura caudalis of the involucrum. This fibula originates at the connecting point of the ventrocaudal end of the lamella externa and the fibula lateralis and it inserts in the caudal portion of the lamella interna either ventrally or dorsally. In the latter case it seems to be replaced by a sinew. Corresponding to its position it may bisect the fenestra ventralis into a fenester ventrocaudalis and ventrofrontalis and/or the apertura caudalis into a apertura caudolateralis and caudomedialis.

• 6. The basal plate is composed of three major parts: pars centralis, pars alaris, and the serrula.

• 7. The pars centralis forms a roof-like structure originating on the basis denticis, on top of the transverse axis, or the dorsum alae, of the wing system. A strong dens medialis forms the ridge of the roof, while groups of teeth form the margins.

• 8. The pars alaris consists of a dorsum alae—the stronger middle part, stretched in transverse direction. On both ends it bifurcates, thus forming five separate areas within the pars alaris: two are paired-the alae frontales and the alae laterales—and the unpaired mediocaudal portion, the tectum caudalis, which is much thinner. These portions seem to correspond to the widely representative five-partition of the alae in basal plates of Gnathostomulida.

• 9. The distal portion of the frontomedial margin of the alae frontales always bears a flat, scale-like dentation: the serrulae. Only in the genus Gnatliostomula does the proximal portion of this margin not end freely; it bends medially underneath the pars centralis. There the two sides meet and form an infundibulum. In this construction the originally paired serrulae continue proximally and fuse medially on the infundibulum.