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 ciliated 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 of Fossil Sponges. 



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 Elasmos- 

 toma and RhapMdonema, whilst Ventriculites cribrosus, Phill., is typically vasiform, 

 and Ventriculites radiatus, Mant., is expanded with a central funnel. Other 



