POLYCMTA—BENHAM. 
87 
In the two tubes, however, from Commonwealth Bay, which agree in externals 
with the latter species, these sponge spicules are present, and their arrangement agrees 
precisely with that described by McIntosh. There is thus a parallel series of form and 
of detail in structure in the tubes of the two “ species.” 
Elders, when comparing the two “ species,” points out that he had no information 
as to whether his specimen of S. mirabilis was obtained in the same haul as his S. spinifera, 
i.e., whether they occurred close together, though they came from the same locality; 
nor had he any information as to the nature of the sea-bottom which would explain the 
difference observable in the structure of the tubes. 
But McIntosh found that the base of some of the tubes of S. mirabilis were em¬ 
bedded in sponges, which would account partly for the small proportion of mud in his 
tube-wall and wholly for the presence of sponge spicules. 
A comparison of the sea-bottom at the localities at which the various specimens of 
S. spinifera have been obtained does not give sufficient information, I think, to account 
for the presence or absence of the spicules. 
The “ Challenger ” S. mirabilis were got on a bottom of “ greensand,” the 
“Valdivia ” S. spinifera from bottoms of “ blue mud,”' of “ volcanic sand” and of 
“ nnid,” and Ehlers states that the tubes were covered with black and grey mud. The 
“ Challenger ” S. spinifera tubes were obtained from “ blue mud.” 
In this recent expedition the tubes came from a bottom of “ granitic rock, 
no ooze.” No mention is made as to whether sponges were found at this station, 
though it is quite possible that this was the case. If so, that would account for the 
spicules in the wall of the tubes. Gravier does not mention whether he examined 
the tubes for spicules, presumably he did not, since they are not mentioned in the 
original account of that species. 
(2) “ The stem of the gill in S. mirabilis in McIntosh’s figure is longer than in 
S. spinifera 
But in the specimens from Commonwealth Bay which had been removed from 
the tube before being preserved, I find that the stem is very short, broad and wrinkled 
(fig. 97), and gives off two approximately equal branches ; it is almost exactly like 
Elders’ figure of S. spinifera. On the other hand, in a specimen which I extracted 
from its tube, within which it had been preserved, the gills are bent backward ; the stem 
is long (fig. 98), as figured by McIntosh for his species. The difference, then, between 
the length of the stem of the gill, as observed by the previous authors, seems to be a 
matter of greater or less contraction. 
In the latter individual its position, flattened against the body and fully extended, 
allows a careful study of its structure to be made. The stem divides into two unequal 
branches, one of which seems to be a continuation of the stem, the other external to it; 
each gives off other branches of varying lengths, which bear the terminal filaments. 
Figure 98 is a careful drawing of the extended gill. 
