538 W. WOODLAND. 
which is otherwise uniformly invested by an extension of the 
scleroblast plasm, which is very difficult to render visible by 
staining, but which is quite evident on decalcification of the 
Cucumarian body-wall. The Cucumarian scleroblast in fact 
very much resembles that of the Echinoid pluteus, which 
also deposits the lime of the skeleton in a kind of ectoplasm 
and not in its internal substance, and differs, in consequence, 
from the scleroblasts of sponges and Alcyonaria, in which the 
deposit is internal. It is a remarkable fact that there should 
exist two such distinct types of scleroblasts as those just 
mentioned, the one in which the lime is secreted in the ecto- 
plasm, and the other in which the lime is secreted in the 
endoplasm, and I know of no explanation of it ; it is possible, 
however, granting this fact, that it explains some features 
of spicule morphology, and this I shall attempt to show 
later. 
In addition to the scleroblasts adherent to the spicules and 
free scleroblasts, there exist two other classes of cells in 
the body-wall of C. sp. and C. brunnea, prepared as 
described above, which must be distinguished from the 
scleroblasts. The more numerous class, universally found, 
and which form agreat impediment to the observation of the 
spicules and their scleroblasts, consists of cells running in 
strands which resemble scleroblasts in every particular save 
their form, which is elongated and drawn out after the 
manner of muscle-cells; I have no doubt but that they are 
the “fibres” already many times described (fig. 37). Similar 
cells—the strands of ‘ endoderm”—are to be found in the 
common Alcyonium digitatum. ‘The other class, compara- 
tively rare, consists of cells about the same size and shape of 
the scleroblasts, but stained an intense opaque green, which 
colour is due to the presence of numerous spherules of a 
substance contained within the cell plasm which readily 
absorbs the lichtgriin. No dark granules are present in these 
green cells, and the nucleus resembles that of the scleroblast 
(fig. 38). I am not certain with regard to their identity ; 
they are not pigment-cells, nor amcebocytes, but they are 
