POLYCH^TA— BENHAM. 87 



In tlie two tubes, however, from Commonwealth Bay, which agree in externals 

 \\'ith 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." 



Ehlers, when comparing the two " species," points out that he had no information 

 as to whether his specimen of S. mirahilis 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 tuljes of S. mimbilis 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'. mirahilis were got on a bottom of " greensand,"' the 

 " Valdivia " >S'. spinifera from bottoms of "blue mud," of "volcanic sand" and of 

 " mud," 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. ' Xo 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, presumalily he did not, since they are not mentioned in the 

 original account of that species. 



(2) " The stem of the gill in S. mirahilis in Mcintosh's ligm-e 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 \\Tinkled 

 (fig. 97), and gives off two approximately equal branches ; it is almost exactly like 

 Ehlers' figure of S. spinifera. On the other hand, in a specimen which I extracted 

 from its tube, within which it had lieeu 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 seen^s 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. 



