SPONGES. 67 



Protozoa. The sponges are simply less altered than other animals, the cells of the 

 inner layer still retain some traces of their original structure, and we have to rate the 

 Poiiferata as intermediate in these characteristics between the Protozoa and the 

 Metazoa. 



The word ' individual ' leads to many serious misconceptions owing to its poj)ular 

 meaning, and we use the word zoon for any whole animal or part of a colony of ani- 

 mals whose structure can be said to embrace the essential characteristics of the grand 

 division or branch to which it belongs. In this sense the single cell is a zoon, with 

 regard to the whole animal kingdom, or when we wish to contrast the Protozoa with 

 the Metazoa. The young sponge, at the period when it has but a simple ccelomatic 

 cavity and one opening, is also a zoon, but it is only a zoon when we wish to consider 

 the Poriferata by themselves. 



We can test this position by comparison with the simplest known forms of sponges, 

 such as the Ascones. The forms of this group have a vase shape, with only one open- 

 ing above, while the pores for admission of water are formed as wanted. The struc- 

 ture and form of this adult sponge is similar to that of the simplest ampullaceous sac, 

 and is also similar to that of the young when the ccelomatic cavity is first formed, and 

 it shows us that all these three forms contain the essential elements of sponge struc- 

 ture, and can thus be appropriately called spongo-zoons. 



After the three layers are fully formed, the ccelomatic cavity extends itself in every 

 direction by the formation of ampullae as outgrowths from its sides, but these out- 

 growths do not carry with them the mesoderm and ectoderm. On the contrary, the 

 outward growth and the formation of a new ampullaceous sac, which is the nearest 

 approach the sponge makes towards the formation of a new zoon, takes place wholly 

 inside of the mesoderm, and the outer layer remains unmodified. This is the case in 

 aU the sponges with a thick mesoderm, and even among the higher forms of c.ilci- 

 sponges. ' Among the primitive Ascones, however, a bud from the side carries with it 

 all the membranes of the body, and is a repetition of the original zoon, a complete bud 

 or ' person.' 



New craters are formed anywhere as the sponge increases in size, by the conjunc- 

 tion of canals of the drainage system and without the slightest signs of budding, and 

 yet Haeckel and others regard each of these craters as a person or individual. The 

 mass may grow out solidly into a branch with a dozen craters, then, according to these 

 authors, it is one dozen small ' persons,' or as it grows out, the dozen small canals may 

 unite and form one canal and one crater, then it is one ' person.' There are plenty of 

 examples in which such variations occur on the same stock, and we think they prove 

 that the accepted ideas of what constitutes an individual or person among the sponges 

 with a thick mesoderm and branching gastrovascular canals are entirely erroneous and 

 founded on the deceptive resemblances of the branches of a sponge to those of other 

 compound forms, which really arise from true buds and are true zoons. 



Haeckel and others have regarded all the vase-shaped sponges as single individuals 

 or zoons, but this seems untenable, except in the group of Ascones. It is not uncom- 

 mon to trace the form of the same species among living sponges from a flattened disc 

 with several craters to a vase shape, the vase being built up by the more rapid growth 

 of the periphery. The inner portion of the ectoderm on top of the animal thus be- 

 comes interna], and the opening above, the crater of one large ' person.' Here the 

 so-called zoon is formed by a transformation which can be clearly proved to be the 

 result of the growth of the external parts. It is evident that the mere fact of the 



