704 
DB. WYVILLE THOMSON ON HOLTENIA. 
The large spicules of the walls are quinqueradiate (Plate LXVI1I. figs. 1, 2, Plate LXIX. 
figs. 2, 4, 5). Four of the rays, which represent the four secondary rays of a hexradiate 
spicule, spread nearly on a plane. They spread, however, irregularly, each branch 
curving independently to suit the plan of structure and the pattern of the particular 
portion of the network in which it is developed. This irregularity is sometimes 
extreme, two of the branches in some cases running nearly parallel with one another, 
while the other two diverge w idely ; and sometimes all the four branches curve round 
and run nearly in one direction ; more usually, however, they form a more or less regular 
cross at their origin (Plate LXIX. figs. 2-4). The fifth ray, which represents one half of 
the primary shaft of a hexradiate spicule, is essentially at right angles to the other four ; 
but this branch is likewise irregular and flexible in its distribution. It usually dips 
down more or less perpendicularly into one of the columns of the sponge-body, which 
it thus serves to attach to the wall. Opposite to the point of junction of the vertical ray 
with the four transverse rays, there is frequently a more or less distinct rounded elevation 
or tubercle. This tubercle undoubtedly represents the sixth ray, the continuation of the 
primary axis of the spicule. In some cases, which occur however more frequently in 
the smaller spicules of this type, the tubercle is developed into a branch, and the spi- 
cule becomes hexradiate, recalling the ordinary hexradiate spicule of the sponge-mass 
of Hyalonema. 
The length from point to point of two of the cross branches of one of these spicules 
is about 15 mm , and the length of the azygous arm from 7*5 to 10. The diameter of the 
ray near the point of decussation is from T to T2. The central cavity in these spicules 
is very distinct. At the point of origin of the rays it divides evidently into five, and 
when the prominence representing the sixth ray is well marked, it sends a short branch 
into it, as in the case of the tubercles on the shaft of some of the long spicules of Ilyalo- 
nema. If the spicule be broken or crushed, the laminated structure so well described by 
Bowerbank in Tethya*, by Max Sciiultze in Hyalonemaf, and by Claus in Euplectella%^ 
becomes very apparent, Ten to fifteen siliceous layers can usually be easily detected. 
Near the point of decussation of the rays these siliceous laminae seem to become further 
separated from one another, and the intervening layers of sarcode thicker. When the 
spicule is burned under the microscope, the black films of carbon which represent the 
burned sarcode may sometimes actually be seen splitting off separate from the trans- 
parent siliceous shells. 
The smaller quinqueradiate spicules (Plate LXVIII. fig. 3) resemble the larger in 
general form ; they are, however, much more delicate and flexible, and even more 
desultory in the distribution of their branches. The stretch of these spicules from tip 
to tip of two rays of the cross is from 8 to 10 mra , and the diameter of each ray not more 
than ‘07 to ’05. Some straight fusiform spicules from *08 to ’5 in diameter, and from 
* A Monograph of the British Spongiadse, Bay Society, London, 1864. 
t Die Hyalonemen. Ein Beitrag zur Naturgeschichte der Spongien. Bonn, 1860. 
f XJeher Euplectella aspergillum (II. Owen). Ein Beitrag zur Naturgeschichte der Kieselschwamme. Mar- 
burg und Leipzig, 1868. 
