No. 3.] DEVELOPMENT OF MARINE SPONGES. 283 
scarcely stains at all and consists of a mere coating for the 
nucleus, prolonged at opposite ends into slender protoplasmic 
processes. Such cells are especially abundant in the mesoderm 
of the dermal membrane, Fig. 4. The edges of pores and 
oscula are always provided with such cells, which here are 
especially long and fibre-like, and serve as a support for the 
free edge. The mesodermic bands which support the dermal 
membrane (Fig. 4), and the partition walls found in branching 
canals (pf. w. Fig. 7) contain great numbers of these fibre-like 
cells. The ectoderm and the epithelioid lining of the canals 
are formed of flat cells. 
The disposition of the peripheral skeletal bundles is shown 
in Fig. 2. The bundles are composed of spicules, such as that 
shown in Fig. 3a (oxytylotes, Sollas), which project slightly 
from the surface of the dermal membrane, Fig. 2. Besides 
forming bundles, the oxytylotes are scattered in abundance 
through the mesoderm, but are not united into a meshwork. 
A variation from the ordinary type of spicule is occasionally 
found, with a head like that shown in Fig. 3 6. After boiling 
in caustic potash, some spicules are always found with the 
pointed end split as in Fig. 3 c, doubtless an effect due to the 
action of caustic potash. The bow-shaped spicules, Fig. 3d 
(toxaspires, Sollas), the s-shaped spicules, Fig. 3 f (sigma-spires), 
and the sigmas, Fig. 3 ¢, are all of about the same size, the 
first form being less abundant than the other two. They are 
found scattered about in the mesoderm in all parts of the 
sponge. 
The large shovel-shaped spicules, of which a face view is 
given in Fig. 3/’a, and a side view in Fig. 3" 4, are rare. 
Shovels of about half the size, of which Fig. 3/4 gives a face 
view and Fig. 3'c a side view, are comparatively abundant. 
When the spicule is viewed more or less from the end, Fig. 3/ a, 
it is seen that the shovel shape is an illusion, that the blade of 
the shovel is not flat, but is a figure of three dimensions. If 
an oval body should be divided by a transverse plane, passing 
through a point on the equator and a point on the opposite 
surface somewhat nearer one of the poles, two parts would be 
obtained, of which the smaller would roughly correspond in 
