Thyonella

Verrill, 1872

4 species

Medium sized to large forms 10-15 cm long contracted. Skin thick filled with spicules. Tube feet in 5 bands and in varying numbers in the interambulacra, depending on the animal's age. Toward the oral and anal ends thickened valves develops, many tube feet are conical, papilliform. Tentacles 10 the 2 ventral smaller. Calcareous ring low but with short posterior prolongations; stone canal small attached to the dorsal mesentery. Polian vesicle single, ventral in position, retractors exceptionally short, attached closely behind the calcareous ring (when tentacles withdrawn. Gonads placed unusually far back in the posterior third of body. Spicules consist of external layer of simple baskets, and an inner layer of 4 holed knobbed buttons of different sizes, also a few larger plates; mostly regularly knobbed. Feet apparently without any end plate. Walls with numerous supporting rods and plates, besides buttons and baskets. Introvert with rosettes, tentacles with large mostly narrow plates and rods which decrease in size toward the tips of the branches. (Deichmann, 1941, for Thyonacta)

The genus was established, rather casually, by Verrill for Pourtalès Colochirus gemmata. The name has been discarded by most authors, but it seems to be the only one suitable for gemmata. The diagnosis has been enlarged so it takes in the members of Thyonacta, Deichmann and provisionally Théel's Thyone pervicax which has become homeless after the name Thyone had been restricted to the species with delicate tables in the skin. (Deichmann, 1954)

Cylindrical to spindle shape. Mouth has 5 flaps. Ambulacral appendages in a double row in the radii, there are a smaller number also in the interradii, in the body middle they are cylindrical and are feet, at the body ends they are conical and are papillae. There are 10 tentacles, 2 ventral ones smaller. The calcareous ring is compact. The hind edge is waved or has short forked tails. In the skin there are cups to the surface, in part deep ones, in part flat or else there is a network plate. Below lie plates which have mainly 4 perforations and bumps which are confined to the middle rod and to the edge. (Panning, 1971, newly conceived)