DEVELOPMENT OF AN APODOUS HOLOTHURIAN 515 
Deep-sea animals, such as the Elasipoda, are, as a rule, highly 
specialized forms. 
2. Becher's work has shown that Rhabdomolgus is essentially 
a synaptid in structure and development. 
3. Edwards' work has shown that tentacles 1-5 in Holothuria 
are not homologus with tentacles 1-5 Cucumaria, but that ten- 
tacles 6-10 are homologous in Holothuria, Cucumaria and the 
Synaptidae. 
4. Ludwig's division of the Holothurioidea into two sub-classes, 
Actinopoda and Paractinopoda, is accepted, since tentacles 1-5 
of the Synaptidae are interradial outgrowths of the hydrocoel, 
and have no homologs in the other holothurians, while tentacles 
1-5 of the pedate holothurians have no homologs in the Synap- 
tidae. 
5. Since the primary outgrowths of the hydrocoel in synaptids 
develop into organs lacking in the Actinopoda, while the second- 
ary outgrowths correspond to the actinopod's primary hydrocoel 
outgrowths, it would seem that the Synaptidae are an older and 
more primitive stock. 
6. The ancestral holothurian is pictured as a pentactula-like 
animal, but provided with radial water-vessels, accompanied by 
double series of pedicels. 
7. The Paractinopoda have arisen from such a form with reten- 
tion of the primary tentacles and loss of radial water- vessels and 
pedicels. The Actinopoda have developed the radial water- 
system and lost the primary tentacles. 
8. The five primary tentacles of the Synaptidae are possibly 
homologous with the dental-sacs of echini. 
9. The ^'atrium" of the pentac tula is possibly homologous with 
the ''amniotic cavity" of echini. 
10. In synaptids the tactile function of the tentacles has been 
specialized to a high degree, while in echini, the homologous 
organs have, through the mechanical function of scraping, been 
developed into teeth.'* 
^ I trust this statement will not be taken at its face value, as pure Lamarckian- 
ism. I personally feel no doubt that natural selection, acting on slight variations, 
has been the real agent at work. 
