192 



ZOOLOGY 



of the body is composed of structureless material containing cells 

 which secrete lime to form the spicules. Eggs and sperms are 

 also developed in this layer and are set free when ripe to develop 

 in the water. 



Spicules may be freed from the living part of the sponge by placing a 

 Grantia in a strong solution of caustic soda for a few minutes. Mount a 

 little of the sediment from the bottom of the dish in water or glycerine. 

 Note the different forms of the spicules. Draw several for your notebook. 



Development of the Sponge. — In the case of the sponge, as in 

 most plants and animals, the life history^ begins with a single cell, 

 the fertilized egg. This cell, as we remember, has been formed by 

 the union of two other cells, a tiny (usually motile) cell, the sperm, 

 and a large cell, the egg. After the egg is fertilized by a sperm 

 cell, it splits into two, four, eight, and sixteen cells; as the number 

 of cells increases, a hollow ball of cells called the blastula is formed 

 (somewhat like the Volvox) ; later this ball sinks in on one side 

 and a double-walled cup of cells, now called a gastrula, results. 



# IP K A # IK 



Stages in the segmentation of an egg, showing the formation of the gastrula. 



Practically all animals pass through the above stages in their 

 development from the egg, although these stages are often not 

 plain to see because of the presence of food material (yolk) in the 

 egg. The gastrula, which swims by means of cilia, soon settles 

 down, a skeleton is formed, other changes resulting in the formation 

 of pores and osculum take place, and the sponge begins life as an 

 adult. The early stages of life when an animal is unlike the 

 adult are known as larval stages; the animal at this time is 

 called a larva. 



The young sponge consists of three layers of cells : those of the 

 outside, developed from the outer layer of the gastrula, are called 



