HYALONEMA (HYALONEMA) PLACUNA. 
213 
figs. 3-8) measure GO-140 p in diameter. Their rays are straight throughout or 
slightly bent in their middle-part, but never markedly curved in their end-part. 
They are 1.4-2. 9 p thick at the base, conic, sharp-pointed, and covered with 
minute and slender, backwardly directed spines, only 0.3 p long. In the middle- 
part of the ray these spines are much more numerous than in the basal and end- 
parts. 
The irregular microhexactines have two opposite longer rays, and four 
shorter rays (Plate 64, figs. 2, 3, 5) vertical to the axis of the two longer. The 
two longer rays are usually fairly equal, the four shorter rays often very unequal. 
These spicules are in form A 120-195 p long and 90-130 p broad; in form B 
110-170 m long and 80-135 p broad. Their rays are similar to the rays of the 
regular microhexactines above described, and are 1.8-2. 5 p thick at the base. 
Transitional forms connect these spicules on the one hand with the regular micro- 
hexactines and on the other with the diactine microhexactine-derivates. 
The diactine microhexactine-derivates (Plate 64, fig. 1) are 220-330 p long. 
Their central tyle measures 4-5 p in diameter. Their two properly developed 
rays are similar to those of the microhexactines and are 2.5-3 p thick at the 
base. The four other rays are reduced, often to quite insignificant protuberances 
of the central tyle. The degree of reduction of these four rays is, in the same 
spicule, usually different, some being generally 5-10 p and more long, while 
others are represented only by slight protuberances of the central tyle. 
The regular microhexactines are 60-140 p long, the ordinary irregular micro- 
hexactines 110-195 p long, a transitional form with reduced rays 13-25 long is 
225 p long, one 10-22 long is 270 p long, and one 2-5 long is 330 p long. 
A comparison of these dimensions shows that the total length of these 
spicules, that is to say the length of their two properly developed rays, is in 
proportion to the degree of reduction of their four other rays. 
This correlation is obviously comparable to that found by me 1 in the 
microscleres of the Tetraxonia, where the number of rays is usually in inverse 
proportion to their size. I am inclined to ascribe this in the case of Hyalonema 
(. Hyalonema ) placuna to the same cause as in the case of the tetractinellids. 
I believe that the cells or assemblages of cells building spicules like the asters of 
the tetractinellids and the microhexactines and microhexactine-derivates here 
under discussion contain a certain and definite amount of potential energy 
available for spicule-building and that this definite amount of energy is expended 
by the spicule-builders in H. (H.) placuna either in producing six smaller more or 
1 R. v. Lendenfeld. Die Tetraxonia. Ergeb. Deutsch. tiefsee-exped., 1907, 11, p. 64. 
