728 THE VOYAGE OF H.M.S. CHALLENGER. 



Suborder I. ACTINELIDA, Haeckel, 1882. 



Definition. — A c an t h o m e t r a with a variable number of radial spines, which 

 are commonly irregularly disposed, not according to the Icosacantha. 



Family XXXIII. Astrolophida, Haeckel. 



Astvolophlda, Haeckel, 1S81, Prodromus, p. 469. 



Definition. — Acantharia with a. variable number of simple radial spines, 

 radiating within a spherical space from one common central point, which is the centre 

 of the spherical central capsule. No lattice-shell. 



The family Astrolophida comprises the simplest and the most primitive forms 

 among all Acantharia, and may therefore ■ be regarded as the common ancestral stock 

 of this whole legion or subclass of Radiolaria. The acanthinie skeleton is composed of 

 a variable number of quite simple radial spines, which are united in the centre of the 

 spherical central capsule and radiate, piercing its walls and the surrounding jelly-veil, 

 within a spherical space. 



The first observed form of this family is the ancestral genus Actinelius, two 

 difi"erent species of which I detected in 1864 in the northern Mediterranean, at 

 ViUafranca, near Nice (compare Zeitschr. f. wiss. Zool., 1865, Bd. xv. p. 364, Taf. xxvi. 

 fio-. 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 Astro- 

 lophus (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, Ij^ing opposite 

 in pairs between the four primary or perradial equatorial spines ; and eight are per- 

 radial tropical spines, lying between the eight interradial tropical spines. Therefore the 



