REPORT ON THE RADIOLARIA. 737 



The family Chiastolida (or Acanthochiasmida) differs from all other ACANTHARIA 

 in the peculiar mode of union of the radial spines ; these are grown together in pairs in 

 the centre of the body, so that every two spines opposite in o'ne axis of the body form 

 together one single diametral spine. All diametral spines are loosely crossed in the middle 

 of the central capsule, or connected by a peculiar screw-like winding, but not united firmly. 



I established the family Acanthochiasmida in my Monograph (1862, p. 402) upon 

 the single genus Acanthochiasma (with three species), in which only ten diametral 

 spines are constantly found ; I derived these from the twenty radial spines of the 

 common Acanthometra, supposing that every two opposite spines of the latter (lying 

 in one axis) were g*rown together in the centre, whilst the intimate connection of the 

 twenty radial spines in the common centre was dissolved. This opinion was afterwards 

 confirmed by Eichard Hertwig, who observed Acanthochiasma intact in the living state. 

 Although two species of this genus are cosmopolitan and very common, the number of species 

 is very small ; I could add to those three older known forms only a single new species. 



Another genus of this family, Chiastolus, was observed by me only in a single 

 specimen, but it is extremely interesting. It has sixteen diametral spines, disposed quite 

 regularly after the same law of the thirty-two spines of Actinastrum which we described 

 above (compare above, p. 729). Therefore we cannot doubt that the former is derived 

 from the latter in the same way, every two opposite radial spines (of one axis) 

 being grown together to form a single diametral spine. As we place Acanthometron 

 (with twenty spines) and Actinastrum (with thirty-two spines) in two different families, 

 it would perhaps be more convenient to separate also Acanthochiasma and Chiastolus as 

 representatives of two different families Acanthochiasmida (with ten diametral spines) 

 and Chiastolida (with sixteen diametral spines). 



As we derive Acanthochiasma from Acanthometron by concrescence in pairs of the 

 twenty radial spines, the Miillerian law of Icosacantha must be employed also to the 

 ten diametral spines of the former, therefore two of them are equatorial, four tropical, and 

 four polar spines. In the same way we may employ the new law of disposition found in 

 the thirty-two radial spines of Actinastrum equally to the sixteen diametral spines of 

 Chiastolus, which we derive from the former, four of them are equatorial, eight tropical, 

 and four polar spines (compare above, p. 732). 



The Central Capsule of the Chiastolida is spherical, and exhibits in general the same 

 shape as in the Acanthonida, and specially in the Astrolonchida. Of course every 

 diametral spine pierces the capsule twice, at two points diametrically opposed. In some 

 species of Acanthochiasma the central capsule is formed very late, so that it seems 

 often to be absent. A very accurate description of the capsule and its nucleus, as well as 

 of the calymma and the pseudopodia, is given by Richard Hertwig in his Organismus 

 der Eadiolarien (1879, pp. 10-18). The pseudopodia are very numerous, and sometimes 

 bear reddish granules. 



(ZOOL. CHALL. EXP. PART XL. 1885.) Rr 93 



