Diagnosis: Tentacles with the stalks cylindrical or terete, not becoming widened distally, either with digits along each side for most of their length (pinnate), or with only one or two digits along each side near the tip (digitate), or without digits at all (simple). Calcareous deposits usually anchors and perforated plates often accompanied by irregular curved rods or minute particles (miliary granules), but any or all of these may be wanting. (Clark, 1907.)

The presence of distinctive anchors and anchor-plates appears to be the most important character for distinguishing this family. However, where calcareous deposits are lacking, the tentacles must be examined. Anchors are also found in the Family Molpadiidae, Order Molpadida, but reference to Plate I, fig. 4 page 4 (Protankyra uncinata) and Plate II, fig. 9 (Heteromolpadia marenzelleri) will show that the two types of anchors are readily distinguishable.

Family Synaptidae contains in excess of 120 species, the majority of which are littoral. There are a few species which live at depths below 250 fathoms. The family is especially well represented in the Indo-Pacific region, where more than half of the known species occur. A single species is at present known from New Zealand waters.