GEODIA MESOTRIAENA. 



103 



and that surrounding the praeoscular tubes, numerous small strongylosphaer- 

 asters, forming a thin superficial layer, are met with (Plate 22, fig. 8). Similar 

 strongylosphaerasters occur in the lower parts of the outer layer of the cortex 

 and in the choanosome. Occasionally I have observed much larger strongylo- 

 sphaerasters apparently transitional to the sterroids. In all parts of the sponge 

 asters with small centra, or with no central thickening at all, are found. In 

 many of these the rays are quite slender and fairly pointed, in others thicker 

 and blunt. Since in all, however, the rays taper more or less towards their distal 

 end and innumerable transition-forms connect the blunt-rayed with the pointed- 

 rayed ones, I think that all these euasters, the blunt-rayed ones as well as the 

 pointed-rayed ones, should be considered as oxyasters. Of these oxyasters two 

 kinds, a larger and a smaller, can be distinguished. The larger kind is restricted 

 to the choanosome. The smaller kind is met with chiefly in the cortex and the 

 distal part of the choanosome. Most of these asters lie superficially in the 

 canal-walls. In the proximal part of the cortex and in the choanosome, chiefly 

 in its distal part, large oxysphaerasters with numerous rays occur. 



The large choanosomal amyhioxes are 4.3-8.2 mm. long and 50-105 ji, usually 

 80-100 [X thick. Those of var. microana are considerably thinner than those of 

 the other two varieties. They are straight or slightly curved and generally 

 sharply and rather abruptly pointed. The two ends are similar (isoactine) or 

 slightly dissimilar (anisoactine). Very rarely spicules of this kind, blunt at 

 both ends, are met with. In one or two this bluntness is such that these spicules 

 might be termed amphistrongyles. 



DIMENSIONS OF LARGE CHOANOSOMAL AMPHIOXES OF GEODIA MESOTRIAENA. 





Var. pachana 



Var. megana 



Var. microana 



Length mm 



5.5-7.1 



5.3-8.2 



4.3-7.8 



Thickness ;i 



75-105 



70-105 



50-77 



The large styles, scattered in small numbers among the large choanosomal 

 amphioxes, are 3-4 mm. long and 70-110 f.i thick. The thickest part of the 

 spicule is close to the rounded end. These spicules are to be considered as 

 amphiox-derivates in which one of the actines has become rudimentary. They 

 are more frequent in var. megana than in the other two varieties. 



The small dermal rhahds of the superficial tufts (Plate 24, figs. l-6a; Plate 

 25, fig. Id) are amphioxes and styles. They measure 380-680 // in length and 

 9-19 }i in thickness. In var. megana (Plate 24, figs, la, 4a; Plate 25, fig. Id) 



