74 SYNOPTIC COLLECTION. 



SlLICEA. 



We cannot pass to the silicious sponges without con- 

 sidering briefly some of the embryological facts relating 

 to their development. The egg of most silicious sponges 

 in the earliest stages is solid * but becomes hollow subse- 

 quently. Later a granular mass accumulates in the 

 interior so that the egg is again solid. The endoderm is 

 formed not by an invagination of a portion of the ecto- 

 derm, but by delamination from the ectoderm, and it is 

 this mass of cells cut off from the ectoderm which fills up 

 the central portion of the young sponge. Both Hyatt 

 and Barrois agree that no gastrula stage exists in either 

 the silicious or the horny sponges. After the appear- 

 ance of the ampullaceous sacs and the spicules, the larva 

 becomes fixed by the collar at the oral end of its body. 

 The canals and pores form and afterward, probably 

 through the mechanical pressure of the water, the cloacal 

 opening breaks through the ectoderm. It will be seen 

 that here, as in the calcareous sponges, this opening is 

 not comparable with the mouth of other animals, but is a 

 secondary formation and in function a cloaca. 



Halisarca( =Oscardla)lobularis O. Schmidt (If. dujar- 

 dini Duj., PI. 67, encrusting a stone), may be one of the 

 simplest of the silicious sponges. Its cells are less differ- 

 entiated than those of most sponges. The ectodermal cells 

 retain their flagella throughout life, and the cells of the mes- 

 oderm are not modified, as in the more specialized forms. 2 

 There is no skeleton, and in the absence of positive 

 information it is possible that Halisarca is one of the 

 primitive forms. Authorities differ with regard to the 

 origin of this genus, and it is at present impracticable to 

 determine whether it is a reduced form, a descendant of 



1 Hyatt, Proc. Boston Soc. Nat. Hist., XIX, 1878, p. 12. 



2 Sollas, Quart. Journ. Micr. Sci., XXIV, 1884, p. 618. 



