152 



PORIFERA. 



arranged as to be traversed by a series of canals, which convey water 

 in and out of the organism, and which are thus connected with 

 respiration and the procuring of food. Looking at the skeleton of a 

 dried sponge, the most obvious sign of the existence of this " aqui- 

 ferous system " is the presence of one or more large superficial open- 

 ings, together with a great number of much smaller apertures (fig. 47, 



%!■/ 



K 



ch 



Fig. 47. — Diagrammatic representation of the structure of a simple Sponge, as seen when cut 

 in half vertically. The general " sponge-flesh " is lightly shaded (_/?) ; the canal-system is black, 

 and the arrows show the course of the water-currents, p fi, " Pores," opening into " inhalant 

 canals," which conduct to the " ciliated chambers " {ch). From these chambers proceed the 

 larger " exhalant canals" (zV), which open into a general central space or "cloaca." This space 

 terminates on the surface by a single large opening or " osculum " (o), which serves for the 

 exit of the water. (After Haeckel ) 



p p). These latter are termed the "pores," and though permanently 

 present in the skeleton, they are only temporarily present in the 

 soft parts, being produced afresh, when required, as openings be- 

 tween the sponge-bells of the ectodermal layer. The " pores " (fig. 

 47> P P) open directly, or through the intervention of more or less 

 extensive subdermal cavities, into a series of canals, which ramify in 



