DEVELOPMENT OF CUCUMARIA ECHINATA 197 
free end, the transverse ring-shaped middle part, and the longi- 
tudinal tubular end opening at the blastopore (PI. 8, figs. 8, 11, 
BD. aril): 
The first part lies perpendicularly to the frontal plane (PI. 8, 
fig. 11 a, ar,), and its posterior end approaches the left side 
(Pl. 8, fig. 12 4, ar,). The frontal plane is determined from 
the position of the stomodaeum, which soon afterwards makes 
its appearance. This part is round in outline, with thickened 
walls near the centre, thinning out towards the periphery, making 
the internal lumen appear usually as a slender dumb-bell shape 
in transverse section. This resembles the feature found by 
Newth in a younger gastrula of C. normani (86, p. 635). 
It is continuous at its postero-ventral end with the second part. 
The second part runs transversely round the dorsal side, 
across the mid-dorsal line, and bends ventrad on the right side 
of the body, slightly turning anteriorly (Pl. 8, figs. 11 B, 12 8, 
ary). The wall is not very thick, the lumen being somewhat 
compressed, with the greater diameter along the body-axis. 
The third part is directly continuous with the second at the 
ventral end of the right limb of the latter, It runs posteriorly, and 
is slightly oblique to the left (Pl. 8, fig. 12 a, ars). In transverse 
section it is round, containing a narrow, often almost obliterated, 
internal lumen (PI. 8, fig. 11 c). The posterior end is continuous 
with the blastopore. 
The internal surface of the archenteron is probably lined with 
cilia all over, though I could not demonstrate their presence in 
sections. 
Selenka (45, p. 170) observed in C. planei that when the 
tip of the archenteron reaches the centre of the blastocoele it 
begins to bifureate. The dorsal branch increases in size very 
rapidly, lying obliquely forward and ventrad, while the other 
ventrally situated branch remains short. This stage is said 
to have been met with on the fourth day. According to Newth 
(36, p. 6385; Pl. 8, fig. 8) the circular flat archenteron of the 
gastrula of C. normani turns to bend in an §$-shape at right 
angles to its plane of flattening, and the anterior flattened sac 
lies obliquely to the body-axis. 
Pp 2 
