GENERAL CHARACTERS. 45 
The late discoveries of Mr. C. Stewart and of Dr. von Lendenfeld also clearly show 
that in many Sponges nerve-cells are present. The skeletal structures of the 
Sponge are also products of the mesoderm, and the siliceous and calcareous 
spicules probably all originate in cells, though their character has not yet been 
definitely ascertained. 
The physiological characters of Sponges have not yet been thoroughly worked 
out. It has long been known that the water containing the food and respiratory 
supplies entered by the pores, and, after circulating through the. incurrent canals 
and the ciliated chambers, found its way to the exterior by the excurrent canals 
and the vents or oscules, but it is not yet certain whether the food is ingested by 
the cells lining the incurrent canals as well as by those of the excurrent 
canals or merely by these latter. The flagellated cells of the cilated chambers 
were formerly believed to ingest the food, but they are now regarded as performing 
respiratory and excretory functions as well as promoting the circulation of the water 
through the Sponge. 
Form or Fosstn Sponcss. 
In regard to their external form, fossil Sponges present the same extraordinary 
variety as living examples of the group. It frequently happens that even when all 
the structural characters of the Sponge have been obliterated, the form remains in 
the fossil state, and until recently this feature has been employed to a great extent 
in the definition of fossil species, though it is now known to be so variable, even in 
the limits of the same species, as to be of very subordinate value. There is no 
connection between the external form and the skeletal characters, for we meet 
with the same variety in all the groups of fossil Sponges, whether lithistids, hexac- 
tinellids, or calcisponges, nor can one particular form be said to be more especially 
abundant than another. 
Fossil Sponges are present in the form of cups, vases, or platters, and transi- 
tional forms between these. Thus, a simple plate-like Sponge may in the process 
of growth become fan-shaped, and by the further infolding of the walls, and the 
anastomosing of their margins where they touch each other, it becomes cup- or 
vase-shaped, or the reverse process may take place, and a Sponge, which in its early 
stages may be vasiform with a funnel-shaped cloacal cavity, becomes expanded in 
the progress of its growth, so that the mature form is that of an expanded platter 
with a small central funnel. As examples of the platter-shaped forms may be 
cited species of the lithistid Verruculina, and calcisponges of the genera Hlasinos- 
toma and Rhaphidonema, whilst Ventriculites eribrosus, Phill., is typically vasiform, 
and Ventricwlites radiatus, Mant., is expanded with a central funnel, Other 
