502 
HUBERT LYMAN CLARK 
ing body-cavity is very distinct. The blastopore has closed and 
the larval gut has bent forward and then backward again. There 
is still no mouth, nor did I find any evidence of a differentiated 
nervous system. The calcareous particles at the posterior end of 
the larva have undergone marked development. The discoidal 
plates (fig. 5 a) have outgrowths on the margin (5 b) which grow 
out rapidly as projecting rays (5 c). The plate is no longer flat 
but somewhat convex at the center, and the rays, instead of lying 
in the same plane with it, curve upward and outward towards the 
surface of the body (5 d). The tips of the rays extend laterally 
(5 e) until they finally coalesce into a solid rim, and thus a fully 
developed wheel is formed (5/). These wheels are about one- 
twenty-fifth of a millimeter in diameter and have about a dozen 
spokes, though the number varies from ten to fourteen; one was 
found with only nine. These wheels are never very numerous and 
they are never aggregated into papillae but are scattered irregu- 
larly in the skin. The remarkable point about them, however, is 
that while there is no known species of Chiridota in which the 
adult has wheels with more than six spokes (normally), in the 
genus Trochoderma the wheels have 10-16 spokes and their devel- 
opment is exactl}^ like those of the larval Chiridota. (Compare 
Theel 77, pi. 2, or Clark '08, pi. 7, figs. 9-12, with fig. 5). 
Ludwig ('92) has pointed out that the wheels of the Myriotrochi- 
nae (to which Trochoderma belongs) differ from those of the Chir- 
idotinae in having a simple, solid hub. The wheels of the larval 
Chiridota have the hub simple and solid. In view of these inter- 
esting facts, it is quite correct to say that so far as the calcareous 
deposits are concerned, Chiridota passes through a Trochoderma 
stage. Semon ('88, pi. II, figs. 5 a-c) has figured the wheels 
found in the auricularia, which he considers to be that of Labido- 
plax digitata; they differ only a little from those found in the larval 
Chiridota rotifera, chiefly in having 16 spokes. It is certainly 
most remarkable if the larva of one of the Synaptinae really devel- 
ops wheels as its first calcareous deposits. The occurrence of 
wheels with many spokes in the larva of Chiridota rotifera has 
raised the question in my mind whether the larva which Semon 
supposed to be the auricularia of Labidoplax digitata was cor- 
