334 THE AMERICAN MONTHLY [Dec 



pyles in the wall of the chambers ; then through the cham- 

 bers and out by exhalent canals into thegastral cavity, 

 and so, finally, to the osculum at the top of the sponge. 

 That, in short, constitutes the canal-system of practically 

 every sponge. Modifications, of course, occur in the differ- 

 ent groups and subgroups. In some, the inhalent canals are 

 long whilst the exhalent, canals are short, and vice versa; 

 in others, the flagellate chambers are spherical instead of 

 cylindrical, being served direct with an inhalent and ex- 

 halent canal. But, in all, the same plan holds — an inha- 

 lent current carrying oxygen and nutrient material and 

 an exhalent current bearing away the water poor in oxy- 

 gen and laden with excretory products of digestion. 



It is easy to conceive how a very much more complica- 

 ted canal-system could have arisen if we imagine the or- 

 dinary cylindrical form to have sent out branches ; and 

 these, in turn, to have also produced branches, giving rise 

 to a colony of Sycon sponges. Further, if we imagine fu- 

 sion to have taken place between contiguous branches and 

 stem, and also a common envelope to have enclosed the 

 branching cylinders, we have presented so intricate a cir- 

 culatory system as almost to baffle interpretation — mil- 

 lions of inhalent pores and hundreds of exhalent openings, 

 such as is familiar to us in the common bath sponge Eu- 

 spongia. 



Before finally leaving the consideration of the canal- 

 system one point of special interest may be mentioned 

 in connection with the flow of water from the flagellate 

 chambers. In some forms (e. g., Grrantia labyrinthica) 

 the exhalent canals are furnished with minute diaphragms, 

 working on the principle of the iris diaphragm. In sec- 

 tions, these are found with varying degrees of aperture 

 showing that under special stimulus the so-called muscle 

 cells which are closely connected with the diaphragms 

 offer some kind of control in closing the apertures for the 

 escape of water from the individual chambers, and so as- 



