PHYLUM PORIFERA 



125 



carry the water, but it substantially increases the surface area of the 

 animal. This seems to be a definite provision to allow increase in 

 volume by keeping the ratio of surface to volume. 



In sponges generally, there are three types of canal systems, identi- 

 fied as the ascon, sycon, and rliagon types of which the first is the 

 simplest, the second intermediate, and the third, the most complex. 

 The canal system of Scypha is of the sycon type. 



The character of the skeleton is a diagnostic feature in the classi- 

 fication of sponges. Some have a skeleton of calcareous spicules, 

 others of siliceous spicules, others of the fibrous spongin, and still 

 others have no skeleton. Spongin of the ordinary bath sponge, which 

 is simply the skeleton of one of these animals, resembles silk chemi- 

 cally. It is formed by some special cells called spongioblasts. The 



honaxon 



.raxon 



Triaxon 

 Hon ax on Jriradiatc 



Fis 



53. — Types of calcareous skeletal spicules found in different sponges. (Drawn 



by Joanne Moore.) 



spicules are of several tj^pes with a number of modifications of each. 

 The monaxon type consists of simple straight spines; the triradiate 

 type consists of those that have three rays joining each other in one 

 plane ; the tetraxon type has four rays radiating from a common point 

 in four different planes ; the triaxon type possesses six rays lying in 

 three axes; and the poly axon type has numerous rays. The cells 

 which produce spicules are known as sclerohlasts. 



The histology of Scypha presents a peculiar arrangement of a 

 number of different types of cells. The outer, dermal layer is com- 

 posed of simple, flat, epithelial cells, contractile cells (myocytes), 

 gland cells wtich secrete the substance for anchorage, as well as the 

 sclerohlasts. A great many of the cells in this layer do not have 

 distinct boundaries, making it a syncytium. In Order Ceratosa, the 



