Ehrenberg, 1875
Solitary polycystines with a siliceous heteropolar shell, which can be represented by several fused spicules only, by a D-shaped ring and associated spines, or by more elaborate, mono- or multilocular latticed skeletons. With the exception of a few forms lacking a well developed skeleton (Zygocircus productus ov; Tetraplecta pinigera ov), the symmetry of this group is characterized by the fact that the two extremes of the major axis define two morphologically different poles of the shell. One of these, conventionally accepted as the top or anterior end, is where the cephalis is located.
A widely recognized, albeit seldom utilized, feature of primary importance for the classification of the Order Nassellaria is the internal skeleton. The internal skeleton consists of a complex set of spines and connecting bars enclosed in the cephalis (main spines/skeleton; main spines/skeleton 2; main spines/skeleton 3), which allow comparison of homologous structures in forms differing widely in their external morphology. Unfortunately, analysis of these features requires dedicated efforts at understanding the complex spatial relationships involved. Furthermore, observation of this internal skeleton is only feasible with well preserved individuals oriented in the right position, which is seldom the case in specimens mounted in permanent slides. In addition to the small scattered perforations typical of the Order Spumellaria, the central capsule of the Nassellaria is usually provided with a single larger pore.
The order comprises the following families:
Family Spyridae
Family Plagoniidae
Family Theoperidae
Family Carpocaniidae
Family Pterocorythidae
Family Artostrobiidae
Family Cannobotryidae