Page:Scientific results HMS Challenger vol 18 part 2.djvu/92

968 The Zygostephanida, constituting the first subfamily, exhibit four large gates only, and differ from all other Coronida in the absence of a basal ring and of basal gates. Therefore the skeleton is entirely composed of two crossed vertical rings, perpendicular to one another; the first is the primary or sagittal ring (inherited from the Stephanida), the second is the new lateral or frontal ring. The four large lateral gates are either quite simple (Zygostephanus) or partly closed by loose and irregular lattice-work (Zygostephanium). The Zygostephanida may be derived directly from the Stephanida by development of lateral branches forming a frontal ring. They commonly possess the same characteristic spines or branches, and the same typical difference between the straight dorsal rod and convex ventral rod of the sagittal ring, which we found in the greater number of Stephanida. The frontal ring is commonly elliptical or kidney-shaped, and much larger than the ovate sagittal ring.

The Acanthodesmida, forming the second subfamily of Coronida, differ from all other members of this family in the possession of a large simple basal gate, surrounded by a simple horizontal basal ring. Only this ring is complete, whilst the two crossed vertical meridian rings (the primary sagittal and the secondary frontal ring) are incomplete, both truncated at the base by the basal ring. Therefore there remain here between the three rings five large gates (recognised previously by Johannes Müller in 1856 in Acanthodesmia): four lateral gates (the same as in the Zygostephanida) and one central basal gate. The latter is always quite simple and serves for the emission of the pseudopodia, arising from the basal pole of the central capsule. The four lateral gates are either quite simple (Coronidium) or partly closed by irregular loose lattice-work (Acanthodesmia). The subfamily Acanthodesmida may be derived either directly from the Stephanida (by development of a central basal gate) or from the Eucoronida (by loss of the basilar part of the sagittal ring).

The Eucoronida, the third subfamily, are the most important group of the Coronida; their numerous species are much more frequent and more widely distributed than those of the other three subfamilies. They may be derived immediately from the Semantida by the development of a lateral or frontal ring. This remains incomplete in the basal part, whilst the two other rings, perpendicular to it (the vertical sagittal ring and the horizontal basal ring), are complete. Therefore the shell constantly exhibits six large open gates between the three rings; four lateral gates (the same as in the Zygostephanida and Acanthodesmida) and two basal gates (inherited from Semantis); the latter correspond to the "jugular pores" of the and ; they remain constantly simple. The four lateral gates may also remain simple (Eucoronis) or they may become partly closed by irregular loose lattice-work (Plectocoronis). The remarkable genus Podocoronis is distinguished by the development of typical descending basal feet, which are regularly disposed and correspond to the typical "cortinar feet" of the other. There may be developed either two lateral feet (as lower