480 
E. W. M ACER IDE. 
I']cliinus and that is the centre for the aboral spike. This 
furnishes uot only the aboral rod but also the postero-lateral 
rods, whilst the dorsal arch, as we have seen, supplies the 
skeleton for the antero-dorsal as well as for the praeoral arms. 
During' the third week the aboral spike begins to be 
absorbed. This absorption takes place at the tip ; the ciliated 
crest is lost and the aboral rod becomes shorter. The absor])- 
tion seems to be carried out by the agency of the cells carry- 
ing the red pigment, for these are seen thickly massed about 
the free end of the spike. The other arms grow longer so 
that the tips of the pr^-oral, antero-lateral, postero-dorsal 
and post-oral arms extend to about the same distance from 
the body taking the web of skin connecting the post-oral arms 
as the base-line. Two large vacuities {vac., fig. 11) appear in 
this web one on each side. The other arms, viz. the antero- 
dorsal and postero-lateral, grow much longer but do not 
attain the length of the rest. The rods supporting all these 
arms become much attenuated so that they actually become 
fiexible. The Echinus-rudiment grows in size and the lobes 
of the hydrocoele begin to appear as tentacles projecting into 
the.amniotic cavity [ten., fig. 10). From the floor of the 
amniotic cavity there grow up other protrusions ; these are the 
rudiments of tlie spines of the adult {ad. sp.). Outside the 
amniotic cavity there begin to appear alternating circles of 
calcareous stars. These are the rudiments of adult plates 
belonging to the aboral region of the adult {ah. p., figs. 12 
and 13). 
On the dorsal surface of the oral lobe an epithelial thicken- 
ing can now be seen. This is almost certainly homologous with 
the organ which I have described (1903) as the larval brain 
in the larva of Echinus esculent us. The corresponding 
organ in E c h i n o c a r d i u m c o r d a t u m shows two outgrowths 
which pass towards the halves of the longitudinal ciliated 
band — a fact which indicates that the probable function of 
the organ is to co-ordinate the activities of the two halves of 
the ciliated band {1. h., fig. 12) 
Towards the end of the third week the Echinus-rudiment 
