1887.] NATURAL SCIENCES OF PHILADELPHIA. 161 



developing and arranging skeleton spicules, and in every way living 

 the life of a young sponge. The foraminal aperture is rarely 

 plain ; more frequently it is infundibular, (PL V, fig. i, a,), having a 

 slightly raised and expanded margin ; while in still other species it 

 is prolonged into cylindrical or funnel shaped tubules (PL VI, fig. 

 iii, iv, and v.) 



In most species, possibly in all under normal conditions, the 

 chitinous coat is surrounded by a "crust" (PL VI, figs, i, ii, etc.), 

 composed of air cells, often so minute as to be with difl^iculty 

 "resolvable," even with a high power of the microscope ; in other 

 species so large as to be readily discerned by the use of a low one. 

 In the first instance it has been called a "granular," in the other, a 

 "cellular" "crust." In this are imbedded (PL V and VI,), the spicules 

 which, as will be hereafter seen, are relied upon to determine the 

 generic classification of these sponges. 



To recur for a moment to the resemblance stated to exist between 

 the fresh water and some of the marine sponges, we can see no 

 obvious reason why all the marine forms should not have their 

 representatives among those belonging to fresh water ; but it is a 

 fact that all of the latter, as yet discovered, are siUcioiis; that is, the 

 skeleton or framework, (corresponding to the elastic fibre of which 

 commercial sponges are composed) upon which the slime-like sponge 

 flesh, kno^vn as "sarcode," is supported, and through whose interstices 

 the currents meander, is composed of silicious spicules, slightly 

 bound together by an almost invisible quantity of firmer sarcode or 

 perhaps of colloidal silica. 



To form the main lines of this skeleton structure the spicules, 

 averaging about i^th. of an incli in length, are fasciculated in 

 bands made up of several spicules, lying side by side, and somewhat 

 overlapping at their extremities ; the crossing lines being formed of 

 more slender fascicles, or even of single spicules. In the different 

 species these "skeleton" spicules vary in size, in the shape of their 

 terminations, and in their more or less spinous character (see Plates 

 VII to XII, a,a,); but while these differences serve, in some degree, as 

 specific guides, they are not sufficiently constant or positive to form 

 a basis for generic arrangement. 



Besides the skeleton spicules, a second class, known as "dermal" or 

 flesh spicules (PL VII to XII, c, d, e, etc.) is found only in some species 

 and in greater or less numbers, either lying ujion the outer "dermal" 

 film or lining the canals in the deeper portions of the sponge. They 



