The Acanthonida, the second suborder of Acanthometra, embraces by far the greatest number in this order, viz., all those forms in which twenty radial spines are regularly disposed after the Müllerian law—Icosacantha (compare above, p. 717). The radial spines of this suborder are either simple or provided with transverse processes (either two opposite or four crossed apophyses). They are commonly united in the middle of the central capsule by their opposed basal ends, forming small pyramids; the meeting triangular faces of the neighbouring pyramids being propped one upon another. Above these small basal pyramids often arises a basal leaf-cross formed by four broad triangular leaves or wings with straight edges; the meeting thin edges of the neighbouring spines serve for strengthening the basal junction and form hollow pyramidal spaces or compartments, filled with the contents of the central capsule (compare p. 721). The suborder Acanthonida comprises three different families, the Astrolonchida, Quadrilonchida, and Amphilonchida. The first family, the Astrolonchida, comprises by far the greater number of the Acanthonida; those genera in which all twenty spines are perfectly equal or nearly equal in size and form. In the second family, the Quadrilonchida, the four equatorial spines are much larger (and often also of another form) than the sixteen other spines (often also the eight tropical larger than the eight polar spines). The third family, the Amphilonchida, is distinguished by the preponderating development of only two opposite equatorial spines, which are much larger (and often also of another form) than the eighteen other spines.
Synopsis of the Suborders and Families of Acanthometra.
| Suborder I. ACTINELIDA.
Number of the radial spines variable, either more or less than twenty, commonly disposed irregularly and not according to the Müllerian law. |
Radial spines very numerous (thirty to a hundred or more), radiating from a common centre within a spherical space, | 1. Astrolophida. | |
| Radial spines between ten and twenty, radiating from one common point within a sphere-quadrant, | 2. Litholophida. | ||
| Radial spines of variable number; every two opposite spines grown together in the centre; therefore numerous diametral spines are crossed freely in the centre, | 3. Chiastolida. | ||
| Suborder II. ACANTHONIDA.
Number of the radial spines constantly twenty, disposed regularly according to the Müllerian law. |
All twenty radial spines nearly equal, and of the same size and form, | 4. Astrolonchida. | |
| Four equatorial spines much larger than (and often also of different form from) the sixteen other spines, | 5. Quadrilonchida. | ||
| Two opposite equatorial spines (or principal spines) much larger than (and often also of different form from) the eighteen other spines, | 6. Amphilonchida. |
