LARVAL SKELETON OF SPATANGUS 48] 
ment in most cases as regards the first appearance of a new 
calcification centre or its subsequent development, &c. 
At the outset I may call attention to the fact that the 
latticed rods, viz. the postoral, postero-dorsal, and posterior 
unpaired (so-called aboral spike), are morphologically different 
from the other simple, though often thorny, rods which serve 
equally as the support of each corresponding arm. Théel 
(11, pp. 40-1) described very clearly the early development of 
the postoral rods of Echinocyamus pusillus as 
follows: ‘they (the latticed rods) begin to arise during the 
gastrula stage as three small processes, one on each rod of 
the star close to its centre, Pl. ii, fig. 38. ‘These processes 
stretch in length, run parallel and become connected by 
transverse beams’. ‘The same is exactly true for the corre- 
sponding rods and algo for the other latticed rods in Spatan- 
gus. In all of these a three-rayed ‘ star’ is first laid down 
lying parallel to the surface of the body. From each of 
the rays or arms, very close to the centre, is given out 
a vertical process, directed towards the surface of the body. 
The latter, three in number if, as in most cases, all developed, 
give rise to a latticed rod. The postoral and postero-dorsal 
rods of Echinocardium cordatum are both stated by 
MacBride to be formed of only two parallel rods (4, 
pp. 475, 477). As compared with the table-hke calcareous 
body, which is commonly met with in all classes of Echino- 
derms, the latticed rod corresponds to the spire, and the three- 
rayed portion to the base. Thus the above-named two-paired 
and ope unpaired latticed rods are morphologically com- 
posite in structure and are from the beginning directed 
vertically to the surface of the body. On the other hand, 
those rods supporting the antero-lateral and postero-lateral 
arms are morphologically simple, being produced either 
as prolongations or branches of the three-rayed base, which 
were lying originally parallel to the surface of the body. The 
body-, recurrent, and horizontal rods are also either prolonga- 
tions or branches of the basal part, which remained running 
along the surface of the body without, however, pushing out 
