68 THE VICTORIAN NATURALIST. 



chambers and out by exhalent canals into the gastral 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 different groups and sub- 

 groups. In some, the inhalent canals are long whilst the 

 exhalent canals are short, and vice versd ; in others, the flagellate 

 chambers are spherical instead of cylindrical, being served direct 

 with an inhalent and exhalent canal. But, in all, the same plan 

 holds — an inhalent current carrying oxygen and nutrient material 

 and an exhalent current bearing away the water poor in oxygen 

 and laden with excretory products of digestion. 



It is easy to conceive how a very much more complicated canal- 

 system could have arisen if we imagine the ordinary 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 fusion to have taken place between con- 

 tiguous branches and stem, and also a common envelope to 

 have enclosed the branching cylinders, we have presented so 

 intricate a circulatory system as almost to baffle interpretation — 

 millions of inhalent pores and hundreds of exhalent openings, 

 such as is familiar to us in the common bath sponge — Euspongia. 



Before finally leaving the consideration of the canal-system 

 one point of special interest may be mentioned and that is in 

 connection with the flow of water from the flagellate chambers. 

 In some forms (e.g., Grantia labyrinthica) the exhalent canals 

 are furnished with minute diaphragms, working on the principle of 

 the iris diaphragm. In sections, 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 

 assist in regulating the current of water flowing through the 

 sponge at large. 



As a kind of working classification based on the canal-system 

 of the calcareous group the following may be suggested because 

 the sponges included in it are easy to obtain locally and are con- 

 venient to manipulate. But, as is well known, the canal-system 

 alone cannot be relied upon to furnish a truly scientific classifica- 

 tion, but it will serve a present purpose in affording a kind of 

 focus in connection with which can be gathered the more im- 

 portant facts which have guided spongologists in adopting a more 

 elaborate arrangement. The attention of those who wish for 

 further information on the classification of this group is directed 

 to the Synopsis drawn up by Professor Dendy and published in 

 the " Proceedings of the Royal Society of Victoria," vol. v. (new 

 series). Subjoined is a simple working classification alluded to 

 above : — 



