60 



THE SIMPLER METAZOA 



much water will the pores of the sponge hold ? Microscopic sections of 

 Grantia are necessary if you are to make out the inhalent pores, the 

 central cavity, and spicules. 



44. Reproduction. — At certain times of the year the 

 sponge reproduces by means of two kinds of cells (eggs 

 and sperms) developed in the middle layer. A sponge 

 may develop both eggs and sperms, but usually develops 

 only one kind at a time. Cells from the middle layer 



move in between cells of the endoderm 

 and grow large and round. These are 

 the eggs (female cells). Other cells 

 move into the endoderm layer and divide 

 into many small ciliated cells (the sperm 

 or male cells). The sperms are set free 

 and escape into the water of the central 

 cavity and out from the body of the 

 parent sponge. A sperm enters the 

 body of another sponge and when it 

 finds an egg, fuses with it, thus forming 

 the fertilized egg. The fertilized egg 

 then begins to grow, and after a definite 

 period breaks away from the parent, 

 moves about for a time, and then settles down, attaches 

 itself, and grows into a mature sponge. The immature 

 sponge has the power of locomotion, but the mature form 

 loses this power. Nevertheless the sponge is an animal. 



Reproduction that comes about through the fusion of an 

 egg and a sperm is called sexual reproduction. The other 

 method of reproduction, called asexual reproduction, also 

 occurs among sponges. By this method, sponges form 

 little buds or branches which develop into new sponges. 



45. Spongilla. — Spongilla (spunj-il'la) is a fresh-water 

 sponge. At the approach of cold weather, certain repro- 

 ductive bodies are formed, known as winter-cells, and 



Figure 59. — Two 

 Stages in the 

 Development of 

 the Sponge. 



