Echinodermata, by Prof. F. J. Bell. 437 
and of allied forms. The marked divergence in the statements 
which I now have to make with regard to Phormosoma pla- 
centa from those made by two brilliant and accomplished 
German naturalists with regard to an apparently allied species 
is sufficient to show this. 
I should add that 1 have made some use of the material 
obtained by H.M.S. ‘ Challenger,’ but the unique condition of 
some of the specimens, the disappearance of the viscera of 
others, and the absence of the remarkable P. rigidum have 
prevented me from making the investigation as complete as I 
wished, 
In the interesting essay on the Echinothurtide *, which Dr. 
P. and Dr. F. Sarasin based on the beautiful form Astheno- 
soma urens, which they discovered off Ceylon, especial atten- 
tion was directed to the organs of Stewart; these are of con- 
siderable size in the Ceylon species. Notwithstanding the 
fact that no description of these organs has been given by 
Thomson or Agassiz, the Doctors Sarasin ascribe to the Echino- 
thuridz as one of their distinctive characters a “ gewaltige 
Entfaltung der Stewart’schen Organe,” and they say, further, 
“Sowohl die Cidariden als die Diadematiden besitzen die 
Stewart’schen Organe, welche bei den Kchinothuriden reich 
entwickelt sind, in rudimentiirer Ausbildung.” I was some- 
what interested to discover how it was that organs so remark- 
ably well developed had not been seen by previous observers. 
The first example I opened served to settle the question on 
the same principle as that on which Tilburina could not see 
the Spanish fleet ; the organs of Stewart were not there to be 
seen. In some anxiety to bring this state of things into 
conformity with the very absolute statement of the Drs. 
Sarasin I opened another specimen ; here I found the arrange- 
ment shown in Pl. XVIII. fig. 2, which is drawn of the 
natural size, the whole test being 110 millim. in diameter. I 
come to the conclusion therefore that in Phormosoma the 
organs of Stewart may be present in a rudimentary or ves- 
tigial condition, or may be absent; I have been unable to 
find any trace of their presence in Phormosoma bursarium or 
P. tenue; but as these specimens have been several years in 
spirit, | will not lay much stress on the apparent absence of 
these organs. I need not do that to show that there is a con- 
siderable difference in the anatomical characters of the two 
genera, differences which most of us have tacitly assumed not 
to exist, which, possibly, we had no reason to expect to see, 
but as certainly no reason not to expect. 
* Ergebnisse naturw. Forsch. auf Ceylon, I. 3. Ueber die Anat. der 
Echinothuriden u. die Phylog. der Echinodermen. 
* 
