GENERAL STRUCTURE OF THE SPONGILLIDÆ. It would be impracticable in this introduction to give a full account of the structure of the Spongillidæ, which in some respects is still imperfectly known. Students who desire further information should consult Professor Minchin's account of the sponges in Lankester's 'Treatise on Zoology,' part ii, or, if a less technical description is desired, Miss Sollas's contribution to the 'Cambridge Natural History,' vol. i, in which special attention is paid to _Spongilla_. The diagram reproduced in fig. 1 gives a schematic view of a vertical section through a living freshwater sponge. Although it represents the structure of the organism as being very much simpler than is actually the case, and entirely omits the skeleton, it will be found useful as indicating the main features of the anatomy. [Illustration: Fig. 1.--Diagram of a vertical section through a freshwater sponge (_modified from Kükenthal_). A=pores; B=subdermal cavity; C=inhalent canal; D=ciliated chamber; E=exhalent canal; F=osculum; G=dermal membrane; H=eggs; J=gemmule.] It will be noted that the diagram represents an individual with a single osculum or exhalent aperture. As a rule adult Demospongiæ have several or many oscula, but even in the Spongillidæ sponges occur in which there is only one. New oscula are formed by a kind of proliferation that renders the structure still more complex than it is when only one exhalent aperture is present. The little arrows in the figure indicate the direction of the currents of water that pass through the sponge. It enters through small holes in the derma into a subdermal cavity, which separates the membrane from the bulk of the sponge. This space differs greatly in extent in different species. From the subdermal space the water is forced by the action of the flagella into narrow tubular canals that carry it into the ciliated chambers. Thence it passes into other canals, which communicate with what remains of the central cavity, and so out of the oscula. The ciliated chambers are very minute, and the collar-cells excessively so. It is very difficult to examine them owing to their small size and delicate structure. Fig. 2 D represents a collar-cell of a sponge seen under a very high power of the microscope in ideal conditions. [Illustration: Fig. 2.--Sponge cells. A=bubble-cells of _Ephydatia mülleri_, × 350 (_after Weltner_). B=gemmule-cell of _Spongilla lacustris_ containing green corpuscles (shaded dark), × 800 (_after Weltner_). C=gemmule-cell of _Ephydatia blembingia_ showing "tabloids" of food-material, × 1150 (_after Evans_). D=collar-cell of _Esperella ægagrophila_, × 1600 (_after Vosmaer and Pekelharing_). E=three stages in the development of a gemmule-spicule of _E. blembingia_ (_after Evans_), × 665. F=outline of porocytes of _S. proliferens_, × ca. 1290: _e_=dermal cell; _n_=nucleus; _p_=pore; _p.c._=pore-cell.]