550 W. WOODLAND. 
Cucumariide, the presence of two masses of endoplasm on 
either side at the middle of the young rod also possibly ex- 
plains why this expands at places when the endoplasm is not 
situated, viz. at the extremities, ultimately leading to bifurca- 
tion—bifurcation, because the sources of material for spicule 
growth are situated at the sides of the rod and not at its ex- 
tremities, and deposition would naturally occur in the direction 
from which the greatest supplies are received. However, no 
such supposition can account for the subsequent bifurcations 
of the four-holed spicule, since in many cases no additional 
cells make their appearance, and if additional cells do arrive, 
either by division of the two or four primary scleroblasts, or 
by the adherence of new scleroblasts, these bear no such re- 
lation to the spicule as would be required either by the sup- 
position just made or by the theory of Hérouard. 
It is possible that the perforations of the plates are in some 
manner related to the necessity for communication between 
the upper and lower strata of the stroma, for it is evident that 
if the spicule plates were all solid, they would, on account of 
their great number, form a serious impediment to the nutrition 
of the external and the respiration of the internal tissues of 
the animal. But a desideratum, and, as I have just implied, 
the perforation of the spicule plate is in all probability a de- 
sideratum, cannot constitute a cause to bring about the de- 
sired variation, and even if it could we are not as yet justified, 
owing to our ignorance of the inheritance of spicule charac- 
teristics, in assuming that the variation would be inherited 
and perpetuated by natural selection. ‘I'he form of the spicule 
is a product of physical causes, but what these causes are I 
cannot yet say. 
On tHE Mopr or DEvosITION OF THE SPICULES IN 
THYONE FUSUS. 
Since completing the work above described on the spicules 
of Cucumaria I have been fortunate enough to obtain from 
