Ixii 
THE VOYAGE OF H.M.S. CHALLENGER. 
Series 1. Spires {(nrelpa, rj, Lat. spira, in plural the twisted coils of a serpent, 
called spires by Milton). 
These spicules have the form of a screw helix, of less or more than one revolution 
and of various pitch. 
1. Sigmaspire (Fig. XII., h, c).^ — A slender rod twisted through about a single revolu- 
tion of a spiral. This spicule has the form of the letter “c” or “ s” according to the 
direction in which it is viewed, and has in consequence been regarded by authors as a 
variable spicule, sometimes twisted in one manner and sometimes in another, but the 
two forms are always associated in the same sponge, and by causing one of them to turn 
round it will be found to assume the appearance of the other, so that there can be no 
doubt as to its spiral form. By making little models of screw spirals with pieces of 
wire the form of this spicule and its different appearances according to circumstances 
will readily be understood. 
2. Toxaspire. — A spiral rod in which the twist a little exceeds a single revolution. 
The pitch of the spiral is usually great and the spicule consequently appears bow-shaped 
when viewed laterally (tricurvate). 
3. Poly spire (spirula, Carter) (Fig. XII., q). — A spire of two or more revolutions. 
It makes but little difference whether we employ the term spirula or polyspire. 
I am myself inclined to adopt Carter’s term since it has priority, and the term 
polyspire is only introduced here to show the relations of this spicule with other 
spires ; not by any means out of that desire for uniformity, which is “ the curse of small 
minds.” 
4. Sigma (Fig. XII., d). — A slender rod-hke spicule curved in the form of the letter 
“ c.” This spicule is not spiral though it probably arises from a sigmaspire by increase 
in size and loss of the spiral twist. . It does not occur in the Tetractinellida. 
The toxa and chela which originate from the sigma : — 
5. Toxa (Fig. XII., p). — A bow-shaped spicule, also without spiral twist. It may 
sometimes develop from the sigma and sometimes from an aster, as in Isops [Caminus) 
apiarium (PI. XLIII. fig. 10). 
6. Chela. — This spicule, which also arises from the sigma, does not occur in the 
Tetractinellida. 
7. Globule (Fig. XII., a) or spherule. — A minute spicule which presents a more or less 
spherical form. This spicule occurs in the Tetillidse associated with sigmaspires ; it is 
also found in other sponges (Caminus sphxroconia), but in them is probably derived 
from an aster by reduction, while in Tetilla it is possibly a primitive form. It is 
placed with the spires for want of a better place, to avoid making a special group 
for it. 
