160 PROCEEDINGS OF THE ACADEMY OF [1887. 



INTRODUCTION. 



A few words of elementary information may be desirable to aid 

 those who for the first time undertake the study of sponges. 



In constitution and general appearence the fresh water sponges 

 resemble many of those of a marine habitat, excepting in one 

 particular. This crucial point is the presence, during certain resting 

 seasons, in most of the former, and the absence from all the latter, 

 of those "seed-like bodies" that have been known and described 

 by various authors under the names of ovaria, gemmules, statoblasts, 

 statospheres, sphserulse, etc. In the past I have generally avoided 

 the use of the familiar word statoblast, as it did not seem clearly 

 proven that the function of these "seed like bodies" of the sponges 

 was identical with that of the statoblasts of the polyzoa etc ; and 

 have used the terms statospheres, or sphserulse, as suggesting 

 merely their general appearance. Latterly, however, I have con- 

 curred Avith several European writers in the use of the old term, 

 gemmules; the principal objection to which, is that with some persons 

 the name may seem like a return to the exploded vegetable theory 

 of sponges. It is hardly necessary to say that this idea is not 

 intended. 



In shape these gemmules are nearly spherical; they are about 

 ■So of an inch in diameter, or as large as very small mustard seeds. 

 They are found sometimes in continuous layers, as at the base of 

 encrusting sponges ; sometimes they rest singly in the interspaces 

 among the skeleton spicules ; again, they occur in groups of a dozen 

 or less, sparsely scattered through the s23onge mass, or in smaller, 

 denser groups, closely enveloped in a compact cellular parenchyma. 

 Their principal coat, presumably of chitin, encloses a compact mass 

 of protoplasmic globules, each of which is charged with numbers of 

 discoidal particles, whose function, though all important, it is not 

 my intention to discuss in the present paper. A circular orifice, 

 rarely more than one, through this chitinous coat, sometimes, 

 though inaptly called the hilum, should be known as the foramen 

 or foraminal aperture. Through it, at the time of germination, the 

 above mentioned protoplasmic bodies make their exit, crawling by 

 an amoeboid movement, and spreading out on every side. In a few 

 hours the infant colony may be seen producing aqueous currents, 



