144 BULLETIN OF THE 
Seen from the actinal side, Pl. VII. fig. 16, it will be noticed that the 
oral opening is very large, and that the rim, ed, of the test surrounding 
this opening is notched. The diameter of the opening is about one half 
that of the test of the sea-urchin. The larger part of this opening is 
closed by a muscular wall, in which are imbedded the teeth and dental 
apparatus of the Aristotle’s lantern, Jan. The five teeth of the last- 
mentioned structure are all well developed. The young of Echinocy- 
amus, figured by Miiller, is probably in about the same stage as the 
young Echinarachnius just described. As far as the form of the spines 
and their position on the test goes there is little question that the 
young Echinoid ascribed by Miiller to Echinocyamus has a somewhat 
different form after the absorption of the pluteus from that of Echinar- 
achnius, although the differences are slight. In both Echinarachnius 
and Echinocyamus we seem to have at first the spines arranged in a 
single row about the margin of the test, an approach to the arrangement 
of spines in Arbacia and some other genera. The young Echinarachnius 
is less spherical than that of Echinocyamus. The forms of the ambulac- 
ral feet are alike. The spines are movable, but their motion is small. 
The motion of the spines in the young Echinarachnius is observed long 
before the absorption of the pluteus. The external changes subsequent 
to the stage last described, passing from the young Echinarachnius into 
that described by A. Agassiz, consist in a diminution in size of the ver- 
tical axis and a migration of the anal opening, ap, more towards the 
margin of the disk. 
The existence of large spines in the young Echinarachnius and _ their 
subsequent diminution in size in the adult may show a likeness of the 
young Kchinarachnius to certain embryonic types where the spines 
attain a relatively large size. The primary position of the centrale and 
subsequent migration of the anal opening from its normal position seem 
to indicate a likeness in the young flat sea-urchin to round forms like 
those which have an apically placed anus. 
The following summary may be made of the preceding observations : 
1. The egg of Echinarachnius can be artificially fertilized, and resem- 
bles that of other Echinoderms in its mode of segmentation. It has no 
polar globules, while the egg is free in the water. 
2. A gastrula is formed by invagination, as in some other Echinoderms. 
3. The pluteus referred to Echinarachnius by A. Agassiz is an imma- 
ture pluteus of Echinarachnius. 
4. The development of the young Echinarachnius on the water-tube 
of the pluteus resembles that of other sea-urchins. The rosette form of 
