110 HENRY BURY. 
more is known of the development of this sinus not much 
importance can be attached to the suggestion. The fusion of 
left anterior and posterior cavities in Asterids makes it rather 
doubtful whether I was justified in asserting (5, p. 37) that the 
axial sinus was a part of the former, though I see that MacBride 
(19, p. 483) accepts my position. In Ophiurids, on the other 
hand, the same authority states (18, p. 135) that the axial sinus 
arises from the posterior cavity, quite distinct from the ampulla 
(anterior cavity). In Echinids I confess I am not sufficiently 
confident in my observations to assert a definite origin for this 
sinus. Further investigation must be left to settle this point, 
which fortunately is not very important to our present inquiry. 
The nomenclature of all these parts is in a great state of 
confusion ; but as long as we use the various synonyms merely 
as names, and not as descriptions of position, not much harm 
will accrue. Seeliger objects to the term “dorsal organ” 
because this structure is apparently more ventral than dorsal 
in Antedon; but what is to be said of the application of the 
epithet “axial,” which he prefers, to Ophiurids and Asterids? 
My hypothesis indicates that this organ may have been at one 
time dorsal, but it certainly is not so in true Echinoderms, and 
this being the case, both terms (axial and dorsal) seem to me 
equally objectionable if held to be descriptive, and equally 
unobjectionable if used merely as convenient names. 
Further Development of Radial Symmetry. 
The ancestral form at which we have now arrived would 
probably, if found in a fossil state, be included in that very 
heterogeneous group, the Cystidea, from the simpler forms of 
which it differs chiefly in possessing only five tentacles. If, 
however, I am right in supposing that it was not fixed, and that 
these tentacles were used for locomotion, it must have been of 
such extremely small size as to render its discovery as a fossil 
very improbable ; for though, for the sake of clearness, I have 
in my diagrams (figs. 45 and 46) made the tentacles more 
slender than is probable, yet it is impossible to imagine an 
animal of any large size supporting itself upon only five of such 
