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



Suborder II. ACANTHONIDA, Haeckel (Pis. 130-132). 

 Acanihonida, Haeckel, 1881, Prodromus, p. 465. 



Definition. A canthometra with twenty radial spines, disposed according "to 

 the Miillerian or Icosacanthan law in five zones each of four spines. 



Family XXXVI. ASTROLONCHIDA, Haeckel (PI. 130). 



Astrolonchida, Haeckel, 1881, Prodromus, p. 465. 



Definition. A canthometra with twenty radial spines of nearly equal size and 

 similar form, disposed according to the law of the Icosacantha. No lattice-shell. 



The family As trol on chid a, the first and oldest of the A can th o n id a, is no 

 doubt the ancestral stock not only of this suborder but also of all A c a n t h o p h r a c t a, 

 i.e., of all Icosacantha, or all ACANTHARIA in which twenty radial spines are regularly 

 disposed according to the Miillerian law, forming five zones each of four alternating spines 

 (compare above, p. 717). The Astrolonchida differ from the Acanthophracta in the 

 absence of a complete lattice-shell, from the other two families of Acanthonida (the 

 Quadrilonchida and Amphilonchida) in the equal size and similar form of all the spines. 

 Probably this equality is nowhere quite perfect, since in all Icosacantha the central 

 bases of the twenty spines exhibit originally certain slight differences of form and 

 junction, effected by the regular disposition itself. But setting aside this slight difference, 

 only recognisable by means of a very accurate investigation of the central junction 

 (and in thinner spines often not at all recognisable), the twenty spines of the Astrolonchida 

 appear perfectly equal. Therefore the four equatorial spines are not distinguished from 

 the sixteen other spines, as is constantly the case in the two following families. 



The number of genera (eleven) and of species (seventy-six) in the Astrolonchida is 

 far larger than in the five other families of Acanthometra, and requires a distinc- 

 tion into three different subfamilies. (A) In the Zygacanthida the form of the radial 

 spines is quite simple, without apophyses or transverse processes ; (B) in the Phracta- 

 canthida each spine bears two opposite apophyses (rarely two longitudinal rows of these 

 opposite apophyses) ; (C) in the Stauracanthida each spine bears a cross of four 

 apophyses, opposite in pairs (rarely four longitudinal crossed rows of apophyses, opposite 

 in pairs). The Phractacanthida and Stauracanthida appear as two divergent branches of 

 the pedigree, derived independently from the common ancestral stock of Zygacanthida. 



In the Zygacanthida, constantly devoid of apophyses. we can distinguish only 

 three genera, characterised by the different fundamental form of the radial spines ; these 

 are: (1) Acanthometron, with cylindrical or conical spines (without edges); (2) Zyga- 

 cantha, with compressed and two-edged spines ; (3) Acanthonia, with four-edged, 

 prismatic or pyramidal spines. The transverse section of the spines is in the first case 



