12 MORPHOLOGY OF INVERTEBRATE TYPES 



the majority of sponges it is not a colonial form, so that indi- 

 viduals produced by budding are connected with the mother 

 individual only at the base. It is devoid of locomotion and 

 remains throughout its life attached to the surface of the rock or 

 pile on which it grows. A great number of calcareous spicules 

 support the soft tissues and play the role of a primitive skeleton. 



General anatomy. Grantia ciliata has more or less the 

 shape of a hollow cylinder with a blind base. It is built on the 

 principle of radial symmetry. The free end terminates in a wide 

 opening or osculum surrounded by a crown of long oscular 

 spicules. Smaller cortical spicules cover the whole surface of the 

 sponge like bristles of a brush. More or less concealed by them 

 are numerous small external pores which lead into the so-called 

 incurrent canals. These canals are arranged radially to the 

 longitudinal axis of the sponge. They end blindly and do not 

 reach the central cavity. This cavity is called the atrium. The 

 surface of the wall, which faces the atrium is also perforated by 

 numerous, regularly arranged pores called apopyles. They lead 

 into radial tubes, which are arranged similarly to the incurrent 

 canals, /. e., radially to the longitudinal axis of the sponge. They 

 too, end blindly, but at the end opposite to the blind end of the 

 incurrent canals, under the so-called cortex or a layer of tissue 

 underlying the external surface of the sponge. The incurrent 

 canals communicate with the radial tubes by means of numerous 

 pores or prosopylcs. Water containing food in the shape of 

 minute animal and plant life is drawn into the incurrent canals 

 through the external pores by the motion of the flagella of the 

 cells lining the radial tubes. From the incurrent canals it passes 

 into the radial tubes through the prosopyles, from the radial 

 tubes into the atrium through the apopyles and is finally thrown 

 out of the body through the osculum. Thus the acts of respira- 

 tion and nutrition are accomplished at the same time. Respira- 

 tion and digestion take place in the radial tubes. 



Miscroscopic anatomy. The walls of the sponge are com- 

 posed of three distinct layers of cells: ectoderm, mesenchyme 



