SPONGES 



103 



noflagellata. They take in and 

 digest food and eject excre- 

 ment from the area inclosed 

 by the collar. The cilia (hairs) 

 by their constant movement 

 create currents which keep the 

 water in motion. Water, then, 

 is taken through the pores into 

 the first or incurrent canals; 

 thence it is passed into the 

 ciliated chambers, and thence 

 into the excurrent canals, and 

 out through large passages 

 terminating in large openings 

 called oscula, or craters. The 

 canal systems vary. In some 

 species they become quite com- 

 plex. 



Sponges vary greatly in 

 shape, size, color, surface, rigid- 

 ity, canal systems, and skel- 

 eton. They are cake-shaped, 

 tubular, digitate, palmate, cup- 

 shaped, vase-shaped, cone- 

 shaped, spherical, hemispheri- 

 cal, pedunculate, etc., their 

 shapes depending upon wheth- 

 er their growth is uniform or 

 is excessive in a horizontal or 

 in a vertical direction. When 

 they grow evenly in both direc- 

 tions massive uniform shapes 

 arise. If lateral growth pre- 

 dominates, broad, low, and in- 

 crusting shapes result. When 

 there is an excess of vertical 

 growth the forms are digitate 5 



Canal system of Sycon gelntinosum. Transverse 

 section through the wall of a cylinder (parallel 

 with the course of the canals), showing one in- 

 current canal (1C) and one radial (R) throughout 

 their length; sp, triradiate spicules; si/, oxeate 

 spicules of dermal cortex (dc) ; sp", tetraradiate 

 spicules of gastral cortex (gc) ; ec, ectoderm ; en, 

 endoderm ; pm, pore-membrane ; pp, prosopyle ; 

 ap, apopyle ; di, diaphragm ; exc, excurrent pas- 

 sage ; PO, paragastric cavity ; em, early embryo ; 

 em', late embryo. (The arrows indicate the course 

 of the water through the sponge.) 



