HOLASCELLA ANCORATA. 39 



Plate 24, figs. 3, 8) the triactines to pentactines are much more numerous than 

 the hexactines. Many of these spicules are very irregular, the rays, also opposite 

 ones, frequently differing very greatly in length, and the longer rays being invari- 

 ably more or less curved. Most of the triactine principals consist of two oppo- 

 site longer rays lying in the same axis longitudinally, and one lateral (transverse) 

 shorter ray, more or less vertical to the rhabd formed by the other two. In 

 some of the principal spicules the rays are not only unequal but seem also to be 

 irregular in position, to enclose angles other than 90° with their neighbours. 

 A closer inspection, however, shows that the axial threads of the basal parts of 

 the rays of such spicules are also regularly disposed at right angles (Plate 24, 

 fig. 8), their apparent irregularity of position being due merely to strong curva- 

 tures near their basal part. The rays are smooth and blunt-pointed. The 

 shorter ones are simply conic and gradually attenuated to the end; in the long- 

 est ones the thickest point often lies a short distance from the base, so that these 

 rays appear somewhat spindle-shaped. Such rays are at the thickest point about 

 7% thicker than at the base. The principal spicules are 18^2 mm. long, their 

 longitudinally extending rays measuring 10-21 mm. in length, their transverse 

 paratangential rays 3-15 mm. The rays are 70-160 n thick at the base. The 

 basal thickness of the rays is, on the whole, proportional to their length. 



The rays of these large principals are, like those of the principal spicules 

 of Holascella taraxacum, composed of a nearly homogeneous axial and a very 

 clearly stratified superficial zone. In the tetractine (Plate 24, fig. 8) the axial 

 zone is 18 M in diameter near the centre of the spicule, whilst the clearly strati- 

 fied superficial zone has here a thickness of 56 ix. The layers of the latter are 

 very unequal in thickness; distally they terminate in cones, the apices of which 

 lie in the axial thread. 



In some of these spicules distinct signs of their having been broken at some 

 time during the period of growth are to be noticed. In the portion of a ray of a 

 principal spicule (Plate 23, fig. 4) a fracture is visible, which shows that the 

 tip of this spicule-ray was broken off at a point where it was about 25 fj. thick, 

 and that the ray continued to grow, not only in thickness but also in length, after 

 this breakage. It is clearly to be seen that a new axial thread, lying exactly 

 in continuation of the old broken one, was formed after the fracture. This new 

 axial thread is widened proximally to a cone, which encloses the tip of the old 

 broken axial thread. The new axial thread is a regenerate, the existence of which 

 shows that the elements attached to the tip of a growing spicule-ray are not the 

 only ones that can build up an axial thread, 



