DEVELOPMENT OF AX APODOUS HOLOTHURIAN 
513 
in the echinoid larva be admitted the importance of this differ- 
ence is somewhat diminished, since the lumens of the dental sacs 
are for sometime continuous with that of the left posterior coe- 
lom. There is, moreover, another feature in the development of 
the young echinoid, which is strikingly hke one shown in the 
development of the pentactula. This is the invagination of an 
ectodermal plate, forming the so-called "amniotic cavity," from 
the floor of which the oral disc of the sea-urchin arises. In all 
essentials this process is similar to the formation of the "atrium" 
in the development of the pentactula, and the relation of the 
radial canals and dental sacs to the floor of the amniotic cavity 
is identical with that of the primary and secondary outgrowths 
of the hydrocoel to the floor of the atrium. 
If the homology of parts, between the young echinoid and the 
pentactula, here suggested, be accepted, we have new proof of the 
close relationship of holothurians and echini. The presence of 
jaws (and hence of dental-sacs) as a fundamental character of 
echini has recentl}' received remarkable confirmation by Mr. 
Agassiz's announcement ('09) that in the very young Echinoneus 
(a simply organized spatangoid) a complete set of jaws is developed 
only to be rapidly resorbed. The presence of five conspicuous 
interambulacral tentacles as a fundamental character of para- 
actinopod holothurians is confirmed by the observations on Chiri- 
dota, published herewith. The assumption that these two sets 
of structures are homologous is purely hypothetical but seems to 
me justified by the facts already known. 
Granting this hypothesis, it follows naturally that in the prob- 
able ancestor of both echini and holothurians, the separation of 
the hydrocoel from the left coelomic pouch did not take place 
until the outgrowths for the radial water-vessels and, alternating 
with them, the rudiments of five primary tentacles, had been 
formed. In other words, the water-vascular system was not a 
closed system but was a part of the body cavity, and the lumens of 
both radial vessels and tentacles were continuous with the body 
cavit3\ echini, however, owing to the formation of heavy 
calcareous deposits (teeth) in these primary tentacles, their 
development has been greatly retarded, and the hydrocoel forms 
and separates from the rest of the coelom before they arise. In 
