PHYLUM PORIFERA. SIMPLE MULTICELLULAR ANIMALS 



99 



cell may take part in the formation of a 

 single spicule. The work required to build 

 a sponge skeleton is almost unbelievable. 

 The silica present in solution in sea water 

 is about P/2 parts in 100,000; hence, to ex- 

 tract an ounce of skeleton, at least a ton of 

 sea water must be drawn through the pores 

 of the sponge and forced out again through 

 the oscula. 



Amoeboid wandering cells 



The amoebocytes (Fig. 47) in the meso- 

 glea between the cell layers in the body 

 wall of sponges give rise to reproductive 

 cells and to several types of somatic cells 

 such as pigment cells, food-storage cells, 

 scleroblasts, and spongoblasts. 



gemmules are the chief means of identifica- 

 tion. A gemmule (Fig. 49) consists of a 

 number of cells from the middle layer of 

 the body wall, which are gathered into a 

 ball, and surrounded by a chitinous shell 

 reinforced by spicules. They are formed dur- 

 ing the summer and autumn. In the spring 

 the gemmules develop into new sponges 

 and are hence of value in carrying the sponge 

 through a period of adverse conditions such 

 as the winter season. 



More than 20 species of fresh-water 

 sponges occur in this country. Spongilla 

 lacustris is the most abundant; it prefers 

 running water. 



ORIGIN AND RELATIONS 

 OF THE SPONGES 



Regeneration 



In many sponges, if an individual is cut 

 into pieces, each piece will grow into a nor- 

 mal animal, a process known as regenera- 

 tion. Cuttings of bath sponges in Florida 

 may increase from IVi cubic inches to llVi 

 cubic inches in two months. The wool 

 sponges of the Caribbean Sea may grow to 

 be IVi feet in diameter. The remarkable 

 regenerative power of sponges is demon- 

 strated when certain species are broken up 

 and strained through fine bolting cloth so 

 as to dissociate the cells; the cells will fuse 

 on the bottom of a dish to form sponge- 

 lets, which in the course of several days 

 acquire canals, flagellated chambers, and a 

 skeleton; and later, they will also develop 

 reproductive bodies. 



Fresh-water sponges 



These all belong to the family Spongilli- 

 dae. They are usually found in clear water, 

 encrusting stones, sticks, and plants, and 

 are often yellow, brown, or green in color. 

 These sponges reproduce by formation of 

 gemmules, and the characteristics of these 



Sponges are many-celled animals in which 

 the somatic cells are somewhat differen- 

 tiated for the performance of special func- 

 tions; that is, division of labor among the 

 somatic cells has developed. Although there 

 is relatively little specialization of the 

 somatic cells, the sponges represent a con- 

 siderable advance over the condition exist- 

 ing even among such complex protozoans 

 as Volvox (Fig. 22). 



Despite the fact that sponges are many- 

 celled animals and contain hints of tissues, 

 there are no organs as in most of the higher 

 animals, and no digestive cavity is present. 

 It is thought that the sponges have devel- 

 oped from some protozoan group, probably 

 the Choanoftagellata. They resemble the 

 colonial protozoans in many ways, such as 

 in the digestion of particles of food within 

 cells, and in the formation of skeletal spic- 

 ules by single cells. They suggest especially 

 certain flagellates that are colonial and pos- 

 sess collar cells, like Proterospongia (Fig. 

 428, p. 605). 



Although the sponges are well enough 

 adapted to their environment to have lived 

 their primitive way of life for millions of 

 years, they do not appear to be in the direct 



