236 - NATURAL SCIENCE [October 
In the most primitive colonial Protist, all the cells of a colony 
are practically alike ; and the colony ultimately breaks up into its 
individual cells, which reproduce in one or other of the ways de- 
scribed above. But in some cases the colonial habit has induced 
differentiation among the cells. There is a striking example of this 
in Protospongia haeckelii (a small organism found in pond-water by 
Savile Kent), which consists of a large mass of cells united by a 
gelatinous secretion. Those at the outside of the mass are provided 
with a waving lash, the base of which is surrounded by a funnel or 
collar of protoplasm. These cells take in the food particles brought 
into contact with them by the waving of the lashes in the surround- 
ing water; while the cells at the centre of the colony appear to be 
only indirectly nourished by the food, which is digested and trans- 
mitted to them from the collared-cells. Our knowledge of the life- 
cycle of the organism is still very incomplete, but it appears certain 
that only the central-cells can truly act as reproductive cells by 
segmentation, while outer-cells may possibly separate to propagate 
the race also by the slower process of nutrition aud growth, followed 
at intervals by simple division. We might almost regard this as a 
Metazoon with two tissues—the outer one nutritive, the inner repro- 
ductive, and ascribe the specialisation to the relative position of the 
two layers: the outer one is favourably situated for obtaining food 
from the ambient water ; while the inner, debarred from all activity 
by its position, and fed and sheltered from the stress of contact with 
the unkind world by the outer layer, devotes its energies to the 
reproduction of the species. 
Indeed, this organism, as its name implies, is, as it were, a fore- 
runner of the Sponges, and probably represents a last survivor of 
their ancestral type. For a simple Sponge is a sack attached by 
the bottom and widely open above, with the wall pierced by 
numerous pores. This wall consists of three layers, an outer 
epidermic layer, an intermediate layer, and an inner or stomach 
layer, the cells of the last possessing lash and collar. The lashes of 
the stomach-cells produce a constant current of sea water through the 
sack, which passes in through the pores and out through the mouth, 
and brings with it the food particles which the stomach-cells 
alone can take up, the two other layers being nourished by them, 
In this case it seems that only such fragments of the Sponge as 
contain all three layers can propagate it; and in nature, indeed, 
hollow outgrowths of the sack are formed as branches, and may even 
be detached as buds. But only the intermediate layer, sheltered as 
it is on every side, differentiates certain cells as reproductive-cells. 
These by brood divisions produce male and female pairing-cells ; and 
the coupled-cell after fertilisation grows up into a fresh Sponge. 
We have here a very marked advance on the primitive colonial 
