478 
E. W. MACBRIDE. 
admit that tlieir first beginnings are entirely independent of 
the origin of the skeleton. 
Important internal changes have meanwhile taken place. 
The coelomic sac on each side has become divided into anterior 
and posterior portions, the anterior situated at the side of 
the larval oesophagus, the posterior at the side of the larval 
stomach. On the left side the anterior division has given 
origin to a posterior thick-walled outgrowth ; this is the 
hydrocoele {hy., fig. 6), the rudiment of the adult water- 
vascular system. This lies just beneath the ectoderm on the 
left side in the concavity between postero-dorsal and post-oral 
arms. An invagination of tlie ectoderm, which will form the 
amniotic ca vity, appears just over the spot where the hydro- 
cele is situated and becomes closely applied to it. The two 
adpressed structures form a compound rudiment which I have 
termed the Echinus-rudiment (1903), and by the growth 
of this rudiment the oral disc of the adult is formed in the 
manner described by me (1903). The larval oesophagus is 
surrounded by a band of circular muscles by the aid of which 
the peristaltic movements involved in swallowing are carried 
on. These muscles [muse, circ., PI. 34, fig. 15) are developed 
from the walls of the anterior divisions of the coelom. The 
larva has now attained a stage in development which in 
exceptional cases may be attained by a larva of Echinus 
esculentus in the same time, but which more frequently 
requires sixteen or seventeen days for its accomplishment in 
the latter species. 
From this point the development of Echinocardium 
and Echinus begin to diverge more and more. For whereas 
in the larvm of the latter genus horizontal crescentic bands 
bearing very strong cilia begin to be differentiated from the 
longitudinal ciliated band, and the aboral ends of all the 
rods of the larval skeleton begin to be absorbed, in larvae 
of Echinocardium neither of these changes take place, 
but on the contrary by the ninth or tenth day a new pair of 
larval arms, unrepresented in the Echinus larva, begin to 
make their appearance. These are the p ostero -lateral 
