72 INVERTEBRATE MORPHOLOGY. 



have been indicated in the preceding description of the canal 

 systems ; the tnesoglcea requires, however, further notice. It 

 consists of a gelatinous matrix which, however, contains large 

 numbers of cells presenting a considerable amount of dif- 

 ferentiation. Some are amoaboid in form, others contain pig- 

 ment, others again are elongated and spindle-shaped, forming 

 the contractile cells, others form the reproductive elements, 

 ova and spermatozoa, while others again are skeletogenous 

 in function, well-developed skeletal structures being present 



FIG. 36. Two FIGURES SHOWING DIFFERENCES IN THE COMPLEXITY OF 



STRUCTURE OF A LEUCON SPONGE. 



A. Leucilla ater (after DENDY); B. Oscarella lobularis (after SCHUJ.ZEK 

 cc = ciliated chamber. p = inbalent pore. sp = spicule. 



in almost all Sponges. In some forms the skeleton presents 

 the form of siliceous spicules either of a simple needle-like 

 form, or presenting modifications of a four- or six-rayed 

 ground-form, or finally assuming the form of hooks, anchors, 

 or spiny spheres. In another group there is, associated usu- 

 ally with needle-like siliceous spicules, a network of a horny 

 material termed spongiolin which forms a supportive scaffold- 

 ing for the soft parts of the Sponge, and lastly, in another 

 group the spicules are composed of carbonate of lime and 

 present a variety of forms (Fig. 36, A). 



