226 
the former. Against this suggestion Jackson raises the objection 
(— besides the multiseriate condition of the interambulacra —) 
that in the Lepidocentridæ there have been no external gills while 
such occur, as well known, in the Echinothurids. It is possible 
that Jackson is right in this assertion, but this cannot be proved 
definitely. The gill notches do not precede the external branchiæ 
in the ontogeny of recent Echinoids, the branchiæ appear before 
the notches, and, as Bather states (''Triassic Echinoderms of 
Bakony”, p. 251), the same course was probably followed in phy- 
logeny. "We cannot therefore expect any definite palaeontological 
evidence as to the origin of the external branchiæ.” "In many 
early species, which all writers agree to refer to the Diademoida, 
these notches are: very feebly developed, and it is quite obvious 
that the branchiæ may have existed without the notches.” Such 
cases are actually found among the Echinothurids (I would refer to 
such figures as Pl. 70.2, Aræosoma thetidis (H. L. Clark), Pl. 77.1, 
Aræosoma leptaleum A. Ag. & C1., Pl. 82.5, Aræosoma gracile (A- 
Ag.), in A. Agassiz & H. L. Clark's Memoir on the Echino- 
thuridæ 7), and upon the whole the gill notches are very indistinct 
and irregular in this family. If Jackson had found such forms 
as those referred to above in a fossil state, he would probably have 
coneluded that no external gills were present. I would by no 
means find it improbable that external gills did really occur in 
some Lepidocentridæ; thus in Hyattechinus pentagonus it is seen 
from the analysis of the test given by Jackson (Pl. 25.1) that 
a naked space occurred at the adoral side of the primordial inter- 
ambulacral plate. One might well suggest that small external gills 
were present here. 
But even if Jackson is right that external gills did not 
exist in the Lepidocentrids, but only "Stewarts organs” ?), I do not 
1) Hawaiian and other Pacific Echini. The Echinothuridae. Mem. Mus. 
Comp. Zool. XXXIV,. 1909. 
?) The suggestion that these organs have the function of internal 
branchiæ” appears physiologically impossible. How could an organ 
