88 



PORIFERA 



[CH. 



termed spicules (Fig. 36, and 6, Fig. 39). In Leucosolenia these 

 are calcareous and have three rays, more or less in one plane a 

 shape technically named triradiate. One limb is usually directed 

 "parallel with the long axis of the tube, and often bears a fourth ray 

 or spine in which case the spicule is quadriradiate. The spicules 

 although remaining unconnected are numerous enough to form 

 a. loose meshwork. 



FIG. 39. Section of a portion of Grantia extusarticulata. Highly magnified. 



From Dendy. 



L. Openings of the inhalant canals. 2. Inhalant canal, 

 inhalant canals into flagellated chamber (prosopyle). 

 chamber. 5. Flagellated or collar cells (choanocytes). 

 7. Exhalant opening of flagellated chamber. 



3. -Openings of 



4. Flagellated 

 6. Spicules. 



The most important points in which the higher sponges differ 

 from Leucosolenia are the folding of the outer and 

 i nner l aver > tne restriction of the choanocytes to 

 small portions of the latter, and the differentiation of 

 the body into distinct regions. 



A common sponge on the British coast, Si/con (Grantia) com- 

 pressum, will illustrate the first step in this complication. This 

 animal has the form of a series of flattened thick-walled upright 

 tubes. The layer lining the central cavity consists of flattened cells, 

 but from this cavity pouches lined by choanocytes extend out into 

 the substance of the wall. These flagellated chambers, as 

 they are often called, communicate with the exterior by a series of 



