14 MORPHOLOGY OF INVERTEBRATE TYPES 



ing spicules. The spicules in Grantia belong to two types. The 

 oscular spicules and the spicules imbedded in the cortex have 

 the shape of longer or shorter needles. Every needle is produced 

 as a calcareous secretion of a single scleroblast. The spicules 

 found in the mesoglcea between the radial tubes and the incur- 

 rent canals are composed of three radii. These triradial spicules 

 are so situated in the wall of the sponge that one of the three 

 rays coincides with a radius and points toward the surface of 

 the sponge. Every triradial spicule is produced as a calcareous 

 secretion of three scleroblasts. 



Reproduction. Asexual reproduction consists in the forma- 

 tion of a bud near the base of the sponge. The bud grows and 

 soon an osculum appears at its free end. In this way loosely 

 connected colonies of several individuals may be formed. 

 Sexual reproduction is preceded by the formation of eggs and 

 sperm cells. Both are modified amcebocytes. In the case of the 

 egg it is a considerably grown and rounded up amcebocyte. 

 In the case of the sperm an amcebocyte is first transformed 

 into a so-called spermatogonial cell from which a number of 

 spermatozoa develop. The fertilized egg remains for a long 

 time in the mesoglcea under the endoderm of the radial tubes. 

 It develops gradually into a ciliated larva or amphiblastula 

 and escapes by way of a radial tube and the osculum. The 

 larva attaches itself with the blastopore end to a suitable sur- 

 face. Later the osculum breaks through at the pole opposite 

 to the blastopore. 



Instructions 



1. Place an alcoholic specimen in a shallow dish with water 

 and examine under the dissecting microscope. Make a quarter 

 page drawing showing general shape, oscular spicules, and 

 cortical spicules covering the surface. 



2. Cut the specimen longitudinally into two symmetric 

 halves. Use for that purpose a razor. Again examine the 

 specimen under dissecting microscope, but this time the cut 



