STUDIES ON THE DEVELOPMENT OF ECHINOIDEA. 313 
cute peristaltic swallowing movements, are developed from the 
inner walls of the coelomic sacs, where they rest against the 
oesophagus; and dilator muscles are represented by proto- 
plasmic strings {dil., PI. 14, tig. 5) which join the antero- 
lateral rods to the outer walls of the coelomic sacs. A larva 
four days old viewed from the dorsal side is shown in fig. 5. 
When the larva has attained the age of six days it has 
increased greatly in size, and rudiments of the remaining 
four arms, viz. the two prm-oral and the two postero- 
doi'sal, are visible as very slight protrusions of the ciliated 
band. Underneath the rudiments of the postero-dorsal arms 
is seen an accumulation of mesenchyme cells, in the centre of 
which a high power of the microscope reveals a minute cal- 
careous spicule — the rudiment of the skeleton of the arm. 
No such accumulation is seen beneath the rudiments of the 
prae-oral arms; their skeleton arises in a later stage, as a 
median dorsal spicule, situated above the oesophagus far 
from the arms, and the actual prm-oral rods are subsequent 
outgrowths from this spicule. This is important, because it 
proves that the outgrowth of the arm is not directly due to a 
mechanical push exercised by the growing arm rod, but must 
rather be due to a chemical influence emanating from the 
spicule. The aboral ends of the body rods become bent 
inwards in a crook-like form (rr., fig. 6), often far better 
shown than in the specimen figured. 
(2) The Development op Echinocaedium Coedatum. 
The egg of Echinocardium cordatnni is not more than 
two thirds the diameter of the egg of Echinus esculentus, and 
the blastula which develops from it and rises to the top about 
eighteen hours after fertilisation is correspondingly small. 
Moreover, it is not spherical, but is elongated along one 
axis more than along the other. It is not, however, regularly 
oval, but would be more correctly described as being of 
cylindrical shape, rounded at the ends. In fig. 7 (PI. 14) one 
of these blastulae is shown. It is a little older than the blastula 
of Echinus shown in fig. 2, and the mesenchyme is more 
