The first observed form of this family is the ancestral genus Actinelius, two different species of which I detected in 1864 in the northern Mediterranean, at Villafranca, near Nice (compare Zeitschr. f. wiss. Zool., 1865, Bd. xv. p. 364, Taf. xxvi. fig. 4). Three other species of the same genus were afterwards found by me in the Challenger collections. Whilst in this Actinelius all radial spines are of the same size, a new nearly allied genus, Astrolophus (with two species), differs from it by the different size of the radial spines, a small number of very large spines being intermingled with a very large number of small spines. In these two genera, Actinelius and Astrolophus (the true "Astrolophida" sensu strictiori), the number of the radial spines is quite indeterminable and their arrangement quite irregular and variable.
A third remarkable genus, Actinastrum, differs from these two genera in the definite number and regular order of thirty-two radial spines, and may therefore perhaps better represent a peculiar family, Actinastrida. In this genus (of which two species were observed) the thirty-two radial spines are disposed in such a regular manner that they lie in four meridian planes, and that their distal ends fall into five parallel zones. These five zones and these four planes are the same as we find in all Icosacantha (compare above, p. [717]). Also the constant twenty spines of these latter are present in Actinastrum; but their number is here enlarged by twelve other spines missing in the Icosacantha; four of these are secondary or interradial equatorial spines, lying opposite in pairs between the four primary or perradial equatorial spines; and eight are perradial tropical spines, lying between the eight interradial tropical spines. Therefore the distal ends of the thirty-two radial spines are disposed regularly in five parallel zones, and while two zones (the two polar) contain only the points of every four spines, three zones (the single equatorial and the two tropical) contain the points of every eight spines. The four meridian planes are in Actinastrum the same as in the Icosacantha, crossed in the spineless axis at angles of 45°. But in the Icosacantha each of the two perradial meridian planes contains six radial spines (two equatorial and four polar), each of the two interradial meridian planes only four tropical spines. Whereas in Actinastrum each of the two primary or perradial meridian planes contains ten spines (two equatorial, four tropical, and four polar), each of the two secondary or interradial meridian planes six spines (two equatorial and four tropical). We find therefore altogether thirty-two radial spines in three orders; eight equatorial, sixteen tropical, and eight polar spines.
Only one other genus of Radiolaria exhibits the same characteristic disposition of thirty-two radial spines as Actinastrum, and this is Chiastolus; but here the two opposite spines of each pair are grown together and form one diametral spine; and the sixteen diametral spines are crossed in the centre of the capsule. In Actinastrum, as in Astrolophus and Actinelius, the central ends or bases of all the spines are pyramidal, and the triangular faces of the neighbouring spines rest one upon another (as in the greater number of Acanthonida). The form of the radial spines in all Astrolophida is quite simple, without lateral processes or apophyses; chiefly cylindrical, more rarely compressed, two-edged or quadrangular.
The central capsule in all Astrolophida is spherical, and in the younger specimens contains a single large concentric and lobed nucleus, but in the older specimens a large number of small nuclei. The surrounding jelly-veil or calymma seems commonly to envelop the spines perfectly. The piercing pseudopodia radiate everywhere between the spines, and are very numerous and thin. The circulating granules in them are sometimes red (Actinelius purpureus).
Synopsis of the Genera of Astrolophida.
| Radial spines of indefinite number and of irregular disposition. |  | Spines of equal size, | 317. Actinelius. |
| Spines of unequal size, | 318. Astrolophus. | ||
| Radial spines thirty-two, disposed regularly in five parallel zones, | 319. Actinastrum. | ||
| Radial spines of indefinite number and of irregular disposition. | ||||
| Spines of equal size, | ||||
| 317. Actinelius. | ||||
| Spines of unequal size, | ||||
| 318. Astrolophus. | ||||
| Radial spines thirty-two, disposed regularly in five parallel zones, | ||||
| 319. Actinastrum. | ||||
Genus 317. Actinelius,[[357]] Haeckel, 1865, Zeitschr. f. wiss. Zool., Bd. xv. p. 364.
Definition.—Astrolophida with a variable and undetermined number of simple radial spines, all of equal size, united in the centre of the spherical central capsule.
The genus Actinelius comprises the most simple and primitive forms among all Acantharia, and may be regarded as the common ancestral stock of this whole legion. The spherical central capsule is pierced by numerous simple radial spines of equal size, the pyramidal bases of which are supported one upon another with their triangular faces in the centre of the capsule. The number and position of the spines are quite indefinite and variable. We may derive Actinelius either from Actissa (Colloidea) by development of acanthinic radial spines, or directly from Actinosphærium (Heliozoa) by formation of a central capsule.