392 WILSON. 
EXPLANATION OF PLATE XVIII. 
(Zsperella fibrexilis.) 
Fic. 47. Part of section through young sponge, showing formation of canal 
wall. =); Immersion 4. 
Fic. 48. Section of young sponge, showing osculum. D.4. 
Fic. 49. Section through peripheral part of young sponge. D.4. 
FIGS. 51, 52, 53. Sections of young sponges, for canal system. C.4. 
Fic. 54. Surface view of young sponge with surrounding ectodermal membrane 
covered with débris. A.4. 
Fic. 55. Combined surface view and horizontal optical section of young sponge 
—note abundance of formative cells and absence of chambers. A.4. 
Fic. 56. Horizontal optical section of peripheral part of young sponge —for- 
mation of peripheral mesodermic zone and canals. D.4. 
Fic. 57. Horizontal optical section of older sponge in which flagellated cham- 
bers have formed. D.4. 
Fic. 58. Surface view of sponge with openings into subdermal spaces. A.4. 
Fic. 59. Part of periphery of preceding figure, to show pores. D.4. 
