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 haeckclii (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 and 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 



