510 
HUBERT LYMAN CLARK 
From our present knowledge of holothurian development, cer- 
tain facts seem to be definitely determined. Of these the most 
important is that the five primary outgrowths of the hydrocoel 
in the Synaptidae, which give rise to the five primary tentacles, 
apparently have no homologs in the pedate holothurians. Asso- 
ciated with this is the fact that the secondary outgrowths of the 
hydrocoel in the Synaptidae are homologous with the radial- 
canals of the pedate holothurians. These outgrowths ( = radial 
canals), moreover, give rise to tentacles 6-10, in the same order 
and from the same radii, in all holothurians. Furthermore, in 
Synaptula, tentacles 11-13 also arise from these secondary out- 
growths, and it seems to me probable that in synaptids with 15 
or more tentacles, the additional ones arise from the same out- 
growths too. As a corollary to these facts we are forced to con- 
clude that the first five tentacles of Cucumaria and Holothuria 
have no homologs in the synaptids. It is, of course, possible to 
imagine that through some kind of inexplicable shifting, the pri- 
mary interradial tentacles of the synaptids have been moved lat- 
erally and then outwardly on to the radial canals in the pedate 
holothurians, and thus to maintain that the primary tentacles are 
homologous in the two groups. There is no evidence, however, of 
such movement and it seems hardly probable; but it is clearly not 
impossible. 
Some recent writers (Perrier, MacBride, Ostergren) have seemed 
to minimize the essential difference between the Synaptidae and 
the other holothurians as revealed by their development, but in 
my judgment this difference far outweighs any of the morpholog- 
ical resemblances between the two groups. The unique character 
of the five primary tentacles of the synaptids is so remarkable 
that Ludwig's classification, by which the Holothurioidea are 
divided into the two subclasses Actinopoda and Paractinopoda, 
ought to be accepted. Since it is the first five outgrowths of the 
hydrocoel which give rise to the primary tentacles of the syn- 
aptids, while the secondary outgrowths ( = radial canals) only 
appear subsequently, it seems to me we must suppose that inter- 
radial outgrowths (tentacles?) preceded radial canals in the 
development of the water-vascular system and that we must re- 
gard the Paractinopoda as the older group. The persistence of 
