HYALONEMA (HYALONEMA) OBTUSUM. 163 



some distinctly anisoactine (Plate 36, fig. 31 ; Plate 39, fig. 38). Their end-parts 

 are thickened more or less to spherical tyles (Plate 36, figs. 31, the left one, 41, 

 44; Plate 39, figs. 34, 35, 36, the right one, 37) or spindle-shaped tyles (Plate 36, 

 figs. 40, 42, 43). These distal thickenings are in var. gracilis usually smaller, 

 rarely (Plate 36, fig. 44, the right one) stouter, than the central tyle. In var. 

 robusta they attain much larger dimensions and have a maximum diameter of 

 90 IX. The middle-part of the spicule is smooth. The two ends are generally 

 covered with spines, for a shorter or longer distance (Plate 39, fig. 38, the left 

 one); they are rarely smooth (Plate 36, fig. 41; Plate 39, fig. 37). 



Although doubtless derived from the tetractines (triactines) among which 

 they occur, these rhabds are hardly at all connected with them by transitional 

 forms, and therefore readily distinguishable from them. 



The acanthophore rhabds, which are to be considered as derivates of the 

 ordinary rhabds of the choanosome, are in var. gracilis (Plate 36, figs. 29, 30, 32- 

 39), where they appear to be more numerous than in the other variety, 0.6-2 mm. 

 long and 10-18 ju thick in the middle. Most of them are rather uniformly curved 

 throughout (Plate 36, figs. 29, 30, 32, 34-36), some are irregularly curved (Plate 



36, fig. 33), and a few strongly angularly bent near the middle (Plate 36, fig. 39). 

 Some of them are fairly isoactine (Plate 36, figs. 30, 37-39), others distinctly 

 anisoactine (Plate 36, figs. 29, 32-36). i\ll these spicules are more or less thick- 

 ened at both ends. In the isoactine forms both terminal thickenings are slight, 

 spindle-shaped, and situated a short distance below the end (Plate 36, figs. 30, 



37, 38). In the anisoactine only one of the distal thickenings is of this nature, 

 the other being stouter, 25-45 m in diameter, spherical or oval, and situated 

 terminally (Plate 36, figs. 29, 32-36). The spindle-shaped thickening usually 

 passes gradually into the body of the spicule; sometimes it is distinctly set off 

 from it (Plate 36, fig. 32). The shaft or body of the spicule is smooth. The ends 

 are sometimes also smooth (Plate 36, fig. 33); usually, however, one (Plate 36, 

 figs. 29, 36) or both (Plate 36, figs. 30, 32, 34, 35) of them bear spines. The axial 

 thread is widened in the spindle-shaped distal thickenings (Plate 36, figs. 37, 38) 

 and extends quite to their end. In the ray-ends thickened to a stouter, spherical 

 or oval terminal tyle, the axial thread does not extend quite to the end. The 

 silica-layers of the isoactine forms therefore appear as tubes open at lioth ends, 

 those of the anisoactine forms as tubes open at one end only. 



An abnormal stout acanthophore 220 fj. in diameter was found in var. gracilis. 

 Its rays are straight, cylindrical, terminally rounded, and very unequal in length, 

 but all about 10 ^ thick. 



The intermediate transitional acanthophores (Plate 39, figs. 1, 6, 11, 12) 



