90 



GEODIA ATAXASTRA. 



2-6 /< long and enclose angles of 33-57°, on an average 47.6°, with the axis of the 

 rhabdome. 



Geodia ataxastra is very rich in asterforms. Eight different kinds of asters 

 can be distinguished. 1. Large oxyasters with few rays. 2. Oxysphaerasters 

 with numerous slender, spiny rays. 3. Oxysphaerasters with numerous stout, 

 smooth rays. 4. Small strongylosphaerasters. 5. Ataxasters. 6. Acanthtyl- 

 asters. 7. Irregular sterraster-derivates. 8. Regular sterrasters. Three and 

 seven are very rare and have been observed only in centrifugal spicule-prepara- 

 tions of var. latana. Six also, which occurs in both varieties, is by no means 

 frequent. The five other kinds, particularly four, are abundant, five being 

 more numerous in var. latana than in var. angustana. One and two, and also 

 foiir, five, and six are connected by some transitional forms; seven may also 

 belong to the series of forms represented by the three latter, and at the same time 

 exhibits great affinities to eight. 



The large oxyasters (Plate 43, figs. 35, 36, 37a, 38; Plate 44, figs. 28c, 29c, 

 30c, 33-35c, 39, 40c) occur in both varieties. Those of var. angustana are 

 usually destitute of a centrum, only the two-rayed forms possessing a slight 

 central thickening. They have from two to ten concentric and regularly dis- 

 tributed, straight or slightly curved, usually simple rays. In some oxyasters, 

 however, one or more of the rays are bifurcate, the terminal branches being 

 nearly equally long and strongly divergent. Very rarely trifurcate rays have 

 been observed. The simple rays are conical, gradually or abruptly pointed, 

 and always spiny. When abruptly pointed and distally covered with very 

 numerous spines they have a somewhat strongyle appearance. As a rule nearly 

 the whole ray is covered with spines, only a narrow belt at the base being smooth. 

 The spines increase in size towards the end, just below which they occasionally 

 form a conspicuous verticil. The spines of the proximal half of the ray are 

 often so minute as merely to render the appearance of this part of it rough. 

 When large enough to be clearly made out, the spines are seen to arise vertically 

 from the ray and to bend down at the end in a claw-shaped manner. In regard 

 to their spines the branched rays agree with the simple ones. The size of the 

 aster is in inverse proportion to the number of its rays. The two-rayed (diactine) 

 asters are 40-50 pt, the three- to seven-rayed 21-40 n, and the eight- to ten-rayed 

 15-28 fi in diameter. The rays are half or a little more than half of the diameter 

 of the whole aster in length, and, at the base, 0.6-2.6 //, usually 1-2 fx thick. The 

 basal thickness is by no means in proportion to the length of the ray, shorter 

 rays being often much thicker than longer ones. As, however, the relatively 



