364 E. W. MACBRIDE. 
of the circle of arm rudiments, though No. I is not quite 
adjusted to hydroceele lobe No. 2, and the hydroceele ring is as 
yet incomplete. Here is a fitting place to give in a word or 
two the gist of Ludwig’s observations on the calcareous plates. 
On the oral side (fig. 17) we notice ten small calcareous stars, 
two at the base of each primary hydroceele lobe, situated on 
the inner side of the first pair of tube-feet rudiments. These 
are the beginnings of the first ambulacral ossicles (amd.). 
On the aboral side we notice eleven plates, one central (C.), five 
situated in the arm rudiments and destined to form the 
terminals (7’.) (the plates which protect the terminal tentacles 
of the water vascular system), and five interradially situated, 
the basals (B.), one of which becomes the madreporite. The 
name ‘ basal” is given on account of an imagined homology 
with the basals of Crinoids ; the groundlessness of this assump- 
tion I shall point out later. All these plates make their first 
appearance simultaneously, rather earlier than Stage F. Fig. 
19 shows the aboral surface of a young star-fish about sixteen 
days old. We see that the anus has been formed close to the 
central; that a plate has been interposed between each terminal 
and the central, the former maintaining its position in the 
tip of the growing arm, and that finally a pair of plates has 
appeared in each interradius, peripherally situated with regard 
to the basals, the latter retaining their position in the centre of 
the disc. These paired interradial plates are homologised by 
Ludwig with the interambulacrals of Echinids. 
Plate 22, figs. 70 and 71, are two sections of a larva of Stage 
G. As in all the figures the stalk is placed as nearly as 
possible in the same position, one can see at a glance the very 
great lateral flexure which the disc has undergone with reference 
to the stalk. We see the relation of the rudimentary larval 
cesophagus to the permanent one ; we further see that the oral 
celom is commencing ventrally to open into the left posterior 
one (this is of course a secondary communication, and I may 
say at once that the oral cwlom does not give rise to a separate 
space in the adult, but merely forms the part of the caelom 
abutting on the inner side of the buccal membrane), and finally 
