238 HYALONEMA (HYALONEMA) GRANDANCORA. 
derivates, are: — -differentiated (distal) ray, 40-75 p long, at the base 6. 5-7.5 n 
thick, and in the middle (together with the spines) 6-8 n thick; lateral rays, 
45-57 n long; proximal ray, when present, about 15 n long. 
The few spicules with only terminally spined rays found in the Palythoa 
armour appear to be quite identical with the corresponding spicules (acantho- 
phores) in the basal part of the sponge. The entirely spined spicules which form 
the bulk of the Palythoa armour (Plate 78 , figs. 20-40) are mon- to tetractine. 
The triactine and tetractine entirely spined forms, which are not numerous, are 
85-164 n in maximum diameter and have rays 20-47 p thick. The much more 
numerous entirely spined monactines and diactines are 90-193 p long, on an 
average 126.7 n, and 24-60 n thick, on an average 43.5 p. A correlation (inverse 
proportion) between their length and their thickness is not indicated. These 
spicules usually appear as stout, terminally rounded rods. They often have 
one or two protuberances which are considered as ray-rudiments. The shortest 
spicules, relatively, of this kind, with rays longitudinally most strongly reduced, 
are oval (Plate 78 , figs. 23-26). The spines are conical and usually about 10 n 
long and broad. The average dimensions (length and thickness) of the monac- 
tine and diactine entirely spined acanthophores 
in the sponge are 122 and 38.6 n, 
in the Palythoa 126.7 and 43.5 ix. 
Thus we see that, although there is no great difference between the two, 
these spicules are somewhat larger, particularly in thickness, in Palythoa than in 
the sponge. A greater difference is found in the average size of their spines, 
which is considerably greater in the spicules of the Palythoa armour, than in 
the corresponding spicules in the sponge. Finally it must not be forgotten that 
the percentage of entirely spined acanthophores is much greater in the Palythoa 
armour than in the sponge. All this shows that the Palythoa does not indis- 
criminately gather and embody the basal spicules shed by the sponge, on the stalk 
of which it grows, but selects and retains only the stoutest and most spiny ones 
as material for building its armour. 
In connection with this I should like to point out that, in the literature on 
the armoured zoanthid colonies living as space-symbionts on the stalks of 
Hyalonemae, their armour is described as consisting of sand-grains only, 1 or 
partly of sand-grains and partly of sponge-spicules and other material, 2 or of 
sand-grains and various other material in their lower part, but chiefly of the 
1 J. S. Bowerbank. On Hyalonema mirabile. Proc. Zool. soc. London, 1867, p. 21, 23. 
2 R. Hertwig. Report on the Actiniaria. Supplement. Rept. Voy. Challenger, 1888, 26, p. 39. 
