Hyatt.] 16 [November 1, 



those organs in the polyps, as compared with the monad-like cells of 

 the sponge. 



Farther than this, the whole arrangement or plan of structure of 

 the organization diverges from that of the rest of the animal king- 

 dom as soon as the spicules and these monad-like cells appear, that 

 is, in the larval stage, and while the embryo is still a locomotive 

 animal. From this period the entii-e development hangs upon the 

 monadigerous layer, as the important structural characteristic to 

 which all others are subordinate. The innumerable pores penetrating 

 the outer membrane or ectoderm, sifting the water pouring into the 

 canals, are advantageous to the organization because they are essen- 

 tial in preventing all but the minutest particles of food from entering. 

 The skeleton serves to support and keep the permanent canals from 

 collapsing from the weight of a large massive mesoderm, or from the 

 thinness of the walls in the fistular forms of Calcispongiae; its place 

 is supplied by a thickening of the endoderm and the close aggregation 

 of the ampullaceous sacs in Halisarca. The arrangement and en- 

 largement of the canals towards the centre, and their final opening 

 into the central cloaca, is equally advantageous for the rapid transit 

 of the vitiated water and excrements of the monadigerous layer 

 through the mass to the cloacal orifices. The mechanical, formation 

 of the latter, and probably of the canals, in great part may be seen 

 in Halisarca. In this sponge the cloacal openings and the entire 

 canal system disappear when the sponge is alarmed by handling. 

 Then the interior is found to be filled with numerous bag-like bodies, 

 which are open at one end and are composed of a thick, apparently 

 tough membrane formed by the ampullaceous sacs, with their contents, 

 as above described. The opening is not closed even in stages of 

 extreme contraction after the laceration of the mass. These internal 

 organs are covered by the thick granular mesoderm which appears to 

 have more than one layer, and above these a distinctly cellular ecto- 

 derm, abundantly perforated when the animal is expanded, but en- 

 tirely homogeneous in a contracted state. When the mass begins to 

 expand the internal openings of the bags are placed in- connec- 

 tion with the exterior by canals opening anywhere apparently by 

 means of the mechanical pressure of the water. The structure 

 of these bags and the Calcispongias, also exhibit the simplest essen- 

 tial elements of sponge form. They show us, that, although the 

 skeleton may be dispensed with, the cellular ectoderm, the meso- 

 derm and endoderm, with its monadigerous lining layer, are abso- 



