DEVELOPMENT OF AN APODOUS HOLOTHURIAN 
509 
appears to be that while Rhabdomolgus is primitive in some par- 
ticulars, it is not to be considered as very near the ancestral holo- 
thurian. From the point of view of holothurian phylogeny, the 
most important facts set forth in Becher's paper are the presence 
of rudiments of radial canals in the adult Rhabdomolgus, accom- 
panied by zigzag rows of 'Hastpapillen, " which very possibly rep- 
resent the remains of pedicels ; the interradial position of the five 
primary tentacles; and the points of origin of the five secondary 
tentacles. These facts are all in accord with the conditions which 
I have found in Synaptula and Chiridota. They show clearly the 
close relationship of Rhabdomolgus to the other Synaptidae and 
their common descent from a pedate ancestor with five primary 
interradial tentacles. There is also indicated the possibility of an 
eight-tentacled and asymmetrical ancestral form. 
The existence of ancestral asymmetry is questioned by Edwards 
('09) who suggests that, since he found in Holothuria floridana a 
different origin for tentacles 1-5, from that found by Ludwig ('91) 
in Cucumaria planci, it will not do to assume an identical develop- 
ment for all pedate holothurians even in the early stages. Ed- 
ward's discovery that the five primary tentacles of Holothuria do 
not arise from the same radial canals as in Cucumaria is of great 
importance, but it is even more interesting to note that tentacles 
6-10 arise in Holothuria from the same radial vessels and in the 
same order that they do in Cucumaria, Synaptula, Chiridota and 
Rhabdomolgus. In other words, while tentacles 1-5 are not ho- 
mologous in the three families concerned, the radial canals and 
tentacles 6-10 are.^ It is further of interest to note as bearing on 
a possible asymmetrical holothurian ancestor, that in all these 
genera, whose development has been studied, the mid-ventral 
radius is precocious and develops its organs earlier than the others. 
- On p. 222 of his paper, Edwards says: "While Clark does not suggest the ho- 
mology, it is possible to regard these 'secondary outgrowths' as the last vestiges 
in the degeneration of the protoholothuroid radial canals." 1 beg to call atten- 
tion to the following passages from my memoir: "the second series corresponds 
to those which in S. digitata give rise to the radial water-canals" (p. 63); "the sec- 
ondary outgrowths of the hydrocoel ring in Synaptidae are homologous with the 
five outgrowths of the hydrocoel ring in the true holothurians" (p. 69). Further 
emphasis on the same point will be found at the bottom of p. 81. It is hardly fair, 
therefore, to say that I did not "suggest the homology." 
