LARVAL SKELETON OF SPATANGUS 483 
concavity at the oral field and has begun to assume roughly 
a tetragonal shape. The star is situated in such a position 
that two of its arms lie bilaterally and the remaining one is 
directed dorsally. The former two ultimately give rise to 
the postero-lateral rods, while the third remains as a short 
but distinet spur-like process all through the larval life 
(figs. 8-8). From each of these arms a vertical process is 
produced, directing posteriorly. These three vertical processes 
form together the aboral spike (fig. 2, ab). Being robust in 
structure the transverse beams extend rapidly so as to 
obliterate the openings between them. 
Hand in hand with the rapid growth in length of the post- 
oral rods the arms of the basal portion develop to assume their 
future position. The body-rods, which run straight postero- 
medially, are the most rapid in growth among them, and their 
posterior ends come to overlap each other (fig. 1, br.). In 
the corresponding stage as well as later, as figured by Krohn 
(2, Pl. vii, figs. 1-8, 6), the posterior ends of the body-rods 
are shown standing fairly apart. Except in a later stage 
where the rods begin to be absorbed at the posterior ends 
(figs. 5 and 7), I have never met with such a state as shown 
in his figures. The second arm, the recurrent rod (re), which 
is at first directed dorsally, soon bends posteriorly. In the 
nteantime it produces a branch at its bent portion. ‘This 
branch, which is the future antero-lateral rod (al), proceeds 
a little towards the median line, but soon bends anteriorly 
to run almost parallel to its fellows of the other side, though 
slightly approaching this as it runs. Its base is a little 
broadened and bears a few minute processes, as shown in 
Krohn’s figure (2, Pl. vii, fig. 5, ¢) and confirmed by 
Mortensen (6, p. 15). The remaining arm of the first 
calcification centre runs along the ventral surface, almost 
transversely towards the median line, but slightly deviating 
anteriorly (fig. 1, vh). This is the ventral horizontal rod. 
The end soon comes in contact with that of its fellow of the 
other side, and they ultimately fuse, forming a characteristic 
thickened joint (figs. 2 and 3, vl). This feature is constantly 
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