352 HYALONEMA (OONEMA) SEQUOIA. 



The S7nall and stout amphioxes (Plate 89, fig. 15b) are centrotyle, fairly- 

 straight, 0.6-2.5 mm. long, and 22-70 n thick near the middle. The proportion 

 of length to thickness is in these spicules 1000 : 17 to 1000 : 31. The central 

 tyle is 24-75 /j. thick, that is 1-7 ^ more than the adjacent parts of the spicule. 



The large amphioxes (tignules) (Plate 89, figs. 1-5) are sUghtly and irregularly 

 curved, not centrotyle, and not exactly cyUndrical in the middle or uniformly 

 thickened toward it; the outline is slightly wavy. They are 5-8 mm. long and 

 100-140 fi thick. The proportion of length to thickness is in these spicules 

 1000 : 15 to 1000 : 20. 



The acanthophores (Plate 85, figs. 9-19) have one to six, most frequently 

 four rays. The diactines are centrotyle. The forms with 



5-6 rays are 140-224 fx in maximum diameter and have rays 10-28 n thick, 

 3-4 " " 95-435 " " " " " " " 15-36" " 



2 " " 212-1050 " long and near the central tyle 14-18" " 



1 ray is 108-180 " " " " " terminal tyle 20-30" " 



The central tyle of the long diactines is usually 5-7 // more in transverse 

 diameter than the adjacent parts of the spicule. In the smaller tetractines the 

 four rays are usually fairly equal; in the larger one ray, or two opposite rays, 

 are often longer than the others. All the long-rayed (diactine) forms and a 

 few of the short-rayed (mon- to hexactine) ones have rays smooth in their proxi- 

 mal and middle-parts and spined only in their end-parts. Most of the mon- 

 to hexactine forms are spined throughout, the terminal spines being, as a rule, 

 considerably larger than the more proximal ones. The spines are vertical, 

 broad, low, conical, and pointed. 



The stalk-spicules are all broken off at the distal end. The parts present 

 have a maximum thickness of 0.2-1.2 mm. 



The microhexactines (Plate 86, figs. 9, 11, 12, 35, 36; Plate 88, figs. 1-4) 

 are 60-200 ix in diameter, generally 95-170 /x, and have equal, regularly arranged 

 rays. The rays are perfectly straight, 4-6 n thick at the base, conical, and sharp- 

 pointed. Everywhere except at the extreme tip they bear spines. The spines 

 on their proximal half arise vertically ; beyond that they incline more and more 

 backward, towards the centrum of the spicule. The largest spines are those 

 arising at a distance of about a third of the length of the ray from the centrum. 

 Here they are about 1.5 ^ long, and from here they decrease in size, both distally 

 and proximally. 



