144 BULLETIN OF THE 



Seen from the actinal side, PI. 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, Ian. The five teeth of the last- 

 mentioned structure are all well developed. The young of Echinocy- 

 amus, figured by MUller, 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 MUller 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- 

 ra] 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 Echinarachnius 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 



