TYPE CCELENTERA. 71 



ing with the cavity of the original cylinder (Fig. 35). The 

 cells lining the central cavity become flattened, the collared 

 cells being found only in the interior of the secondary cylin- 

 ders M^hich radiate from the central chamber {cc). Pores, 

 termed prosopyles [pr), occur in the walls of the secondary 

 cylinders, which are closed at their free ends. Through these 

 prosopyles water passes into the interior of the radiating cyl- 

 inders, thenoe into the central cavity and so to the exterior 

 by the osculum. Further complication occurs by the walls 

 of the radiating cylinders coming in contact with each other 

 and fusing in a more or less irregular manner, the space be- 

 tween the various cylinders being thus divided into a series 

 of more or less well-defined inhalent canals (tc) into which the 

 water passes through pores {p) which lie, morphologically, be- 

 tween the extremities of the radiating cylinders. The cavities 

 of these cylinders now form the ciliated chambers, and Sponges 

 in which they possess the cylindrical form are said to belong 

 to the Sycon type. The annexed diagram (Fig. 35) illustrates 

 the different stages of complexity met with in Sycon Sponges. 



The next complication consists of the branching of the 

 ciliated chambers, though they still retain a cylindrical shape, 

 and their separation from the central cavity by a tract lined 

 with flattened cells (Fig. 36, A) ; and finally the collared 

 cells become limited to a portion of the radial chambers, 

 the ciliated canals thus becoming circular in shape and united 

 with the central chamber by long and rather slender canals 

 lined with flattened cells (Fig. 36, B). ' This constitutes what 

 is termed the Leucon type of structure. In- this type the 

 pores upon the surface of the Sponge frequently do not open 

 directly into the canals leading to the ciliated chambers, 

 but into a wide lacunar cavity, the subdermal space, lying be- 

 low the cortical layers of the sponge, and with this the canals 

 communicate. 



To these complications of arrangement further complexity 

 is added by the occurrence in many Sponges of what maybe 

 considered budding, in some cases leading to the formation 

 of definite branches, or in others producing only a number 

 of oscula, each, however, with its own canal system. 



The general characteristics of the ectoderm and endoderm 



