DEVELOPMENT OF CUCUMARIA ECHINATA OTT 
last on the left side of the mid-ventral radial canal (‘Text-fig. 9B, 
I, I’); this attains a size equal to the other four as late as 
the seventh day. Becher (8, 1908, p. 4) attributes this differ- 
ence between Cucumaria and Holothuria to the higher 
tentacle number of the latter in the adult. This must not be 
rashly concluded since the feature in question has not yet been 
recorded in other many-tentacled forms such as Phyllo- 
phorus, Pseudocucumis, &e. Apart from the differ- 
ence in the sequence of their appearance, the distribution of 
the primary tentacles with regard to the radii does not differ 
very much in Cucumaria and Holothuria. Indeed, 
the two agree in the circumstance that the tentacles alternate 
with the radial canals as pointed out by Newth (p. 639). 
The only notable difference is that in Cucumaria each of 
the pair lying in the dorso-lateral interradii is supplied by 
a tentacular canal sent ventrad from the dorso-lateral 
radial canal of each side, while in Holothuria the corre- 
sponding tentacles are supplied by branches sent dorsad 
from the ventro-lateral radial canals. Now, if we are right 
to admit that the primary tentacles all originate directly from 
the ring canal, but not from radial canals, such difference as 
found between these two genera does not seem to me so great 
and fundamental. As will be shown later, in Cucumaria the 
dorsal pair of radial canals grow faster than the ventral pair, 
so that the bases of the tentacles in question shift dorsally and 
at last become branches of the dorsal pair. In Holothuria, 
on the contrary, the ventral pair of radial canals, bemg more 
vigorous in growth than the dorsal pair, might have conquered 
in pulling together the bases of those tentacles. As a matter of 
fact, in Holothuria the ventro-lateral radial canals are 
in adult state generally more strongly developed than the dorsal 
pair, while in Cucumaria these two pairs differ very little. 
Another common feature in both cases is that two of the 
primary tentacles belong to the mid-ventral radial canal. It 
is a noteworthy fact that even in such a form as C. echinata, 
whose ventral pair of tentacles are markedly smaller than the 
remaining eight in the adult stage, the former are represented 
NO. 258 Q 
