STUDIES ON THE DEVELOPMENT OF ECHINOIDEA. 315 
elongated outgrowth from the archenteron, which becomes 
nipped off and then divided into right and left vesicles. Spots 
of dark red pigment have been developed. But it is the 
skeleton which especially arrests our attention. The post- 
oral arms are already developed, and each is supported by two 
parallel rods connected with each other by numerous cross- 
bars. The antero-lateral arms have not yet appeared, but the 
antero-lateral rods are already formed — each a single cal- 
careous rod. From each of tliese rods a branch (r. r.) pro- 
jects backwards, this is the rudiment of the recurrent rod. 
The body rods are well-developed single bars. Between their 
aboral ends an accumulation of mesenchyme cells can be seen, 
in the centre of which a tiny calcareous star [ab) can be 
seen. This accumulation of cells represents the 
formative matrix of the skeleton of the aboral 
spike — a structure characteristic of the Spatangoid lai’va and 
not found in tlie larva of any regular Echiuoid. During the 
course of the next day the antero-lateral arms sprout out and 
so does the club-shaped aboral spike. The recurrent rod has 
grown backwards parallel to the body rod which it now equals 
in length; this rod is usually vestigial in Echinus escu- 
lentus, but is shown on one side in the larva represented in 
fig. 6. But in Echiuocardium cordatum the recurrent 
rods extend to the aboral pole and here fork ; the dorsal fork 
unites with its fellow, whilst the ventral forks unite the ends 
of the body rods, and in this way a terminal ring is formed in 
every way comparable to the so-called “ frame” at the aboral 
end of the Sphaerechinus larva. The skeleton of the aboral 
spike [ah) consists of three slightly diverging rods connected 
with each other by cross-bars. At their -n,n ter ior ends they 
join the terminal ring, which has just been described. One 
of these rods is dorsal and median, the other two are lateral. 
It follows that the aboral spike possesses a most complicated 
skeleton. 'I’he apex of the aboral spike is covered with a 
crest of long-ciliated cells [cil., figs. 10 and 11). During 
the course of the next day the rudiments of the postero-dorsal 
arms make their appearance and in each there is a tiny star. 
