STUDIES IN THE EMBRYOLOGY OE ECHINODERMS. 441 
latter ; but this, as already mentioned, sometimes occurs in the 
Pluteus. A large dorso-central is also present. 
Besides these five basals several other plates are formed on 
the left side of the Pluteus ; at first I imagined that all these 
were related to the hydrocoel, but closer examination of them 
has led me to suspect that some of them should be regarded 
as developed round the left enteroccel ; their numbers and 
positions are, however, extremely difficult to determine, and at 
present I cannot attempt to describe them. The spines 
belonging to some of them are shown in fig. 11. 
Hoi othurians. — The only plates in this group which are 
known to be present in the bilateral larva are the “ Kalkrad- 
chen ” and plates of the water-vascular ring in Auricularia, 
but as none of these can be homologised with plates in the 
other groups, it is useless to pursue the matter further at 
present. Our knowledge of the skeleton of other Holothurian 
larvae is sadly deficient. 
Summary and Conclusions. 
Not only has it been shown in the foregoing pages that 
many skeletal plates are developed in the bilateral larva 
(Dipleurula), and that they bear a definite relation to the 
body- cavities, but the discovery that the terminals lie on the 
left side enables us to establish a typical bilateral form from 
which all the conditions found in existing larvae may have been 
derived ; this typical form has five radial and five interradial 
plates on each side, in definite relation to the body-cavities, as 
shown in the following Table, in which are also given the 
names by which the plates in question are usually known. 
r 
Position . . 
l 
Right Enteroccel. 
Left Enteroccel. 
Radial 
Interradial 
Radial 
Interradial 
Name . . . £ 
Primary 
Radials 
Basals 
Terminals. 
| 
Orals. 
