THE PORIFERA. 109 



the young Flhrospongia appears to be developed out of the 

 inner cells, which, in the mean while, have become spiculiger- 

 ous. However, the details of the mode of development of the 

 FibrospongicB require further elucidation. 



In both the marine and fresh-water sponges the ingestion 

 of solid matters — such as carmine and indigo — by the mo- 

 nadiform endodermic cells has been seen by several observ- 

 ers. According to Haeckel, the solid particles, which usually 

 are taken in between the flagellum and the collar, may also be 

 ingested at other parts of the surface of the endodermic cell. 

 In the course of such experiments, also, granules of the pig- 

 ment may be found in the ectoderm, but, whether they enter 

 it directly or secondarily from the endoderm, is unknown. 

 Sponges absorb oxygen, and give off carbonic acid with great 

 rapidity ; and the manner in which they render the water in 

 which they live impure, and injurious to other organisms, sug- 

 gests the elimination of nitrogenous waste matter. 



The syncytium may contract as a whole, and is liable to 

 local contractions, as when the oscula or the pores shut or 

 open. The contours of the cells of which it is composed are 

 invisible in the fresh state, and hence it appears as a mere 

 " sarcode " or transparent gelatinous contractile substance, 

 in which nuclei and granules are imbedded here and there. 

 But Lieberkuhn has shown that, when the water in which 

 Spongilla lives is heated to the point at which thermic coagu- 

 lation of the protoplasm of the cells occurs, their boundaries 

 at once become defined, and the cells commonly detach them- 

 selves from one another. The syncytium is therefore formed 

 by the close union, and not by the actual fusion, of the cells 

 of the body. 



It is a very interesting fact that thread-cells, similar to 

 those which are so abundant in the Cmlenterata, are said to 

 occur in some sponges. Eimer ^ finds these structures in 

 species of the Renierinoe. The thread-cells are scattered 

 through both endoderm and ectoderm, and abound on the 

 free surface of the former, where it limits the canals of the 

 sponge, but do not occur on the outer surface of the ectoderm. 

 The same observer states that he found partly digested re- 

 mains of small crustaceans in the ventricular cavities and 

 passages of both silicious and calcareous sponges. 



The Porifera present three principal modifications — the 

 Myxospongki?^ the Calcis2)ongi(jp, and the Fihrospongide — the 



1 " Nesseizellen und Saaraen bei See-Schwaramen." (ArcMv fur Mlhro- 

 skovUche Anatomie, viii., 1872.) 



