54 PAL^OTEOPUS JOSEPHIN-.^. 
There is little difference in the arrangement of the ambulacral plates in 
older or younger specimens. The two large genital openings, already present 
in the youngest specimens known, have greatly increased in size, and the 
inadreporic openings are better developed in larger specimens (PI. XXIII. 
Fig. 12). The mode of formation of what has been called a compact abacti- 
nal system is admirably shown in the structure of the apex of Paleeotropus, 
in which the exterior sutures of all the plates adjoining the interambulacral 
areas are most distinct, while the interior junction of the genital plates has 
become completely obliterated (PI. XXIII. Fig. 12), they being still sepa- 
rated by the odd imperforate genital plate which is intercalated between 
them, and of which the posterior and lateral sutures are still well marked, 
Avhile the interior anterior sutures are no longer visible. In Pala^obrissus 
the sutures of all the plates of the apex can be distinctly detected. 
In the youngest stages, as well as the larger, we find near the actino- 
stome in each ambulacral area one or two unusually elongate sphteridia. 
They have a comparatively long reticulated stem, and it is interesting to 
note that in some of the larger specimens we find near the ambitus on the 
actinal side peculiar miliary spines which I am inclined to consider as modi- 
fied sphferidia. These peculiar spines have the shaft of a miliary, but the 
tip is swollen out to a clear sphere resembling in all respects the head of 
sphoeridia, of which tlie base of attachment is usually merely rudimentary, as 
described by Loven ; in addition we have such types as the spho3ridia typical 
of the actinal side of the genus Palseotropus. This would seem to show 
that the sphasridia, like the pedicellariae, the spines of the fascicles, and 
perhaps other appendages of the test of Echini, are only modified forms of 
spines. 
Danielssen and Koren * have figured peculiar spines of Ilymenaster, 
which also throw considerable light on the nature of the sphan-idia. They 
are clusters of reticulations at the base of a central sliaft, much like the 
reticulations figured by Loven as characteristic of the base of spha?ridia. 
These spines are surrounded by a bag-like pouch representing undoubtedly 
for Starfishes the bag-like appendages of certain spines of Asthenosoma and 
Aspidodiadema, Avhich in their turn seem to have close affinities w'ith the 
peculiar gland of certain Echinid pedicellariaj described by Sladen and 
others, as I have already noticed in the description of some remarkable 
spines occurring on the test of Aspidodiadema. 
* Plate II. Figs. 2, 3, Nyt. Slug, for Niitiirvid., XXVIII. 
