SPONGES. 49 



Branch II. — PORIFERATA. 



The sponges are even now popularly regarded as plants, although for many years 

 naturalists have recognized them as members of the animal kingdom, while the investi- 

 gations of the past fifteen years have shown them to be animals of by no means the 

 lowest type. In the preceding pages we have seen that the unicellular Protozoa do 

 not reproduce by means of eggs, but by a process of division or segmentation, resulting 

 in a varying number of embryos, germs, or spores. All of the higher animals, includ- 

 ing the sponges, are composed of multitudes of cells, each performing its own part in 

 the economy of the individual, and while reproduction by division is frequent in cer- 

 tain groups, all have recourse to specialized cells or eggs for the perpetuation of the 

 species. On account of these differences all multicellular animals have been collec- 

 tively termed Metazoa, in contradistinction to the single-celled Protozoa. There is 

 here a similar relationship to that which exists between the spore-bearing and the seed- 

 bearing plants. In an egg-beaiing animal there is a specialization of some of the cells 

 of the tissues and parts to form the male and female reproductive elements, just as in 

 the flowering plant there is a similar specialization of the tissues and leaves to form 

 the male and female products and the organs of reproduction, and as the latter by the 

 union of the sexual elements form fertile seeds, so in the Metazoa the union of the egg, 

 or female element, with the spermatozoan, or male reproductive jjroduct, produces a 

 fertile egg. 



In the Poriferata the development of the sexual elements appeaVs in a simple 

 form ; parts or cells of the tissues within the body of the same sponge grow larger 

 than the rest, and become eggs while other cells change into spermatozoa. The 

 sponges are, therefore, hermaphrodites, and besides they have no external genital or 

 reproductive apparatus and no special apertures for the extrusion of the young. It 

 has been found, however, that some sponges are female, or at least produce few if any 

 sperm-bearing cells, and these sj)onges in some cases die soon after giAdng birth to 

 their broods of young. In most sponges self-fertilization seems to take place ; indeed, 

 such would ajifiear to be the inevitable necessity since the male and female elements 

 are enclosed in the same membranes. 



Sjjonges are all aquatic, are found in the waters of every part of the globe, and in 

 suitable locations may be exceedingly abundant. So far as known they are all seden- 

 tary animals, constrained with few exceptions to pass all but the earliest stages of 

 their existence fastened to the same submerged object to which they became attached 

 in their early youth. The young possess powers of locomotion and can seek out new 

 places of abode, but the adults must remain in one place and take whatever of food or 

 fortune the passing currents may bring them. Thus they can only live and flourish in 

 places where there are floatitig clouds of microscopical plants and animals, and their 

 spores. These form their staples of subsistence and must come to them as the rain 

 comes to the plant. They can use for the recej^tion of food only the upper and 

 lateral surfaces of the body, the lower, attached surface, being of course unavailable 

 for such purposes. To this rule there are some excejJtions. For instance, Suberites 

 compacta, a sand sponge, has no base of attachment and is apparently capable of living 

 with either side uppermost; there are also some wanderers, sponges which have 



VOL. I. — 4 



