390 WILSON. 
EXPLANATION OF PLATE XVII. 
(Esperella fibrexilis.) 
Fic. 33. Ectoderm of posterior pole and adjoining parenchyma of swimming 
larva — maceration product. 
Fic. 34. Rosette group of sigma spicules from swimming larva—seen in 
optical section. 
Fic. 36. Longitudinal section through incompletely metamorphosed larva. D.4. 
Fic. 37. Vertical section of sponge attached to surface film of water. D.4. 
Fic. 38. Vertical section of recently attached sponge. A.4, tube out. 
Fic. 39. Peripheral part of section through recently attached sponge. D.4. 
Fics. 40, 41. Parts of sections similar to the preceding, showing anlages of 
flagellated chambers. D.4, tube out. 
Fic. 42. Part of section through recently attached sponge, showing formation 
of canals and flagellated chambers. D.4. 
Fic. 43. Part of section through young sponge, the mesoderm of which con- 
sists almost entirely of fine cells. D.4. 
Fic. 44. Vertical section through young sponge — subdermal cavities, canals 
and chambers completely differentiated. D.4. 
Fic. 45. Vertical section through a young sponge. DA. 
Fic. 46. Group of multinucleate mes-entoderm cells. 
Fic. 50. Section of a young sponge — for canal system. C.4. 
