ON THE RADIOLARIA AS AN ORDER OF THE PROTOZOA. 377 
It is the prevalent opinion, originated by Muller, and adopted 
by all later writers without exception, that the spines of the 
Acanthometrce are tubular , and that processes of the sarcode 
pass through the tubules and show themselves at their apices in 
the shape of pseudopodia. This, I have long since tried to show, 
is altogether an error, resulting from the high refractive quality 
and the imperfectly siliceous nature of the material of which they 
are composed in this family alone of the Protodermata , and in 
one or two curious free-floating Polycystina ; the illusory ap- 
pearance of tubularity being caused by the longitudinal ribs or 
flanges which are present in most spines, and when looked at, 
under the microscope, on one or other of the faces upon which 
they naturally take up their position, give the appearance of 
the spines being hollow. Just in like manner in certain genera 
the spines are apparently truncate and cleft in half at their ex- 
tremities by a deep angular notch like the letter V. But so far 
from this being, as alleged by Muller and others, a cleft, the V 
is formed by the rectangular spines having a small portion of 
their ends cut off from opposite angles in such wise that a 
“ knife-edge ” is produced diagonally from corner to corner ; 
the intensely hyaline nature of the material preventing the 
knife-edge being seen, except under the best illumination and 
under the use of very high magnifying power. 
I may remark also that it was long ago pointed out by me 
that the reason why the spines of the Acanthometrce , which 
abound in the surface waters of the ocean, are never found in the 
deep sea deposits, is that owing to this imperfectly siliceous 
character, they become dissolved before they reach the bottom. 
This opinion has also been very recently adopted by Sir Wyville 
Thomson. (See Figs. 7, 8, 26, 26 a , 26 b and 27.) 
Of the Thalassicollidce , it is unnecessary for me to give 
any further description here than I have already given in the 
body of my paper ; inasmuch as they are, under any circum- 
stances, a very aberrant group ; and any attempt to picture their 
characters is calculated, in my opinion, rather to increase than 
diminish the difficulties which surround the history of these 
lower forms of animal life. 
The Dictyochidce are also an aberrant family, to this extent, 
that their siliceous framework is invariably, and in every part, 
tubular. It consists also invariably of two perfectly distinct but 
symmetrical and equal-sized basket-like portions, within the 
cavities of which the bulk of the body-substance is contained, 
and by it protected. A Dictyocha may therefore be regarded 
as a symmetrically formed Plagiacantha or Acanthodesmia ; 
with this difference, that the framework of each individual is 
tubular, like a sponge spicule, and always composed of two 
symmetrical and equal pieces, instead of one unsymmetrical one. 
