DEVELOPMENT OF CUCUMARIA ECHINATA 199 
teron has been lying (PI. 8, fig. 144, st). These changes very 
much resemble the mesenchyme-formation and the invagination 
process oceurring in the late blastula. Only in this place does 
Ludwig’s opinion, that the ectoderm shares in the mesen- 
chyme-formation, seem to be true. 
This author observed in the third-day larva of C. planci 
that the stomodaeum appeared on the ventral side immediately 
behind the pre-oral hood (22, p. 606). According to Newth 
(86, p. 634), the stomodaeum appears in C. saxicola and 
CGC. normant in forty-eight hours, i.e. on the middle of the 
third day, as a crescentic invagination at the junction of the 
opaque and loss opaque regions. The horns of the crescent extend 
backwards and ultimately fuse up and the enclosed area sinks in. 
It lies very obviously to the left of the mid-ventral line as deter- 
mined by the pedicels. Similarly a crescentic depression appears 
in the second-day embryo of H. floridana, according to 
Edwards (12, p. 213), but it gradually deepens and straightens, 
growing out to either side until it extends entirely across the 
ventral surface. The plane in which the groove lies is at an 
angle of 50° with the sagittal plane of the adult holothurian. 
11. DreLEURULA. 
Under the term * dipleurula ’ I mean the stage which connects 
the gastrula with the barrel-shaped larva or doliolaria. This 
stage is characterized by remarkable changes occurring in the 
archenteron accompanied by a rapid increase of the mesenchyme 
cells. As seen from the exterior the larva has become slightly 
shorter than in the foregoing stage, the stomodaeum has 
appeared, and it differs from the next stage in having no cillary 
bands. This stage is passed during the thirtieth to fortieth 
hours, i.e. from the end of the second day till early in the 
morning of the third. 
Ludwig (21, pp. 274-5) suggested that there mght be 
a stage, reminding us of the auricularia, during the changes 
which take place between the gastrula and the barrel-shaped 
stage. In his study of C. planci (22, p. 606) he pointed out. 
the fact that the buccal cavity has at the begmning a garland- 
