246 E. W. MACBEIDB. 



of the circle of arm rudiments, though No. I is not quite 

 adjusted to hydroccele lobe No. 3, and the hydrocoele 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 hydroccele lobe, situated oa 

 the inner side of the first pair of tube-feet rudiments. These 

 are the beginnings of the first ambulacral ossicles {amb.). 

 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 XV, 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 

 oesophagus to the permanent one ; we further see that the oral 

 coelom 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 coelom does not give rise to a separate 

 space in the adult, but merely forms the part of the coelom 

 abutting on the inner side of the buccal membrane), and finally 



