LARVAL SKELETON OF SPATANGUS 485 
and fuse midway between the body- and recurrent rods 
(figs. 4 and 6,c). There are very often some irregular spines 
or branches from the dorso-ventral connexion thus formed. 
As the result of this connexion there is formed a rectangular 
framework as seen from side (cf. Mortensen, 5, p. 75, 
Pl. ix, fig. 9). From the point where the antero-lateral rod 
diverges from the recurrent rod there is formed frequently 
a short process directed anteriorly (figs. 2 and 6). This seems 
to have no significance. 
Lastly, at the end of the two-armed stage the body-reds 
fuse at the point where they have been overlapping each other, 
so as to form an oblique cross. Very often there is formed 
an accessory connecting-span between the two body-rods. 
This is a short transverse piece lying a short distance anterior 
to the crossing-point. Now the caleareous framework 
encircling the stomach has become fairly rigid. The body-, 
recurrent, and ventral horizontal rods of both sides are fused 
in the median plane with the respective fellow of the other 
side, while, on the other hand, the body- and recurrent rods 
are connected with each other near the posterior end on each 
side of the body. 
After having reached this state the body-rod increases no 
longer in length, so that, as long as its posterior end remains 
unabsorbed, its length can be taken as unit in describing the 
dimensions of other parts. The length of the body-rod can 
easily be measured when the larva is laid with its ventral side 
downwards, so that the rod is seen in its real length without 
foreshortening. As expressed in terms of the ratios to the 
body-rods, the post-oral rod reaches during the two-armed 
stage a leneth more than twice as long as the body-rod, the 
antero-lateral rod more than one-half, and the aboral spike 
about one-third. 
The aboral process and the antero-lateral arms become 
discernible almost simultaneously. It may now be called the 
four-armed stage (figs. 8-5). The change which takes place 
during this stage is the enormous increase in lengths of the 
post-oral and antero-lateral rods and of the aboral spike. 
K k2 
