system of internal channels and feeding chambers, 

 for the bleaching of the skeleton has destroyed all 

 the delicate fibers that supported the actual canals. 

 In the simplest sponges the feeding cells completely 

 line one vase-shaped internal cavity, while in com- 

 plex sponges like the bath sponge they are restricted 

 to innumerable little special feeding chambers inter- 

 posed between the incurrent channels and the excur- 

 rent ones. In either case the cells that trap the food 

 particles as they go by are also the cells that produce 

 the food-bearing currents. These important and al- 

 most unique cells are called "collar cells" because 

 they have, at the free end that projects into the 

 cavity of the chamber, a single long, hairlike fla- 

 gellum surrounded at its base by an erect and deli- 

 cate protoplasmic collar. Food particles brought to 

 the cell by the beating of the flagellum adhere to the 

 sticky collar and slide down its outer surface to be 

 engulfed by the cell. The many collar cells all feed 

 quite independently, so that a sponge cannot digest 

 any particle too large to be taken into a single cell. 

 No matter how massive the sponge, its diet is re- 

 stricted to minute organic particles, bacteria, micro- 

 scopic algae, the smaller protozoans, and the many 

 eggs and other single sex cells set free in the water 

 by various plants and animals. 



Sponges constitute the simplest of the well-defined 

 groups of many-celled animals, and the only one in 



which the largest opening into the body is not a 

 mouth and the feeding machinery has entrances that 

 are less conspicuous than the exits. They are also 

 alone among many-celled forms in having collar 

 cells, though there are collared flagellates among the 

 protozoans. For these reasons, and others besides, 

 the phylum Porifera is removed from the ranks of 

 the Metazoa (p. 15) and set aside in a separate sub- 

 kingdom of animals, the Parazoa. 



In quiet shallow seas one can sometimes detect a 

 "boiling" of the water where the outgoing current is- 

 sues in a jet from a large sponge. If the edge of a 

 vent is struck sharply, it may be seen to close slowly, 

 perhaps almost imperceptibly. There may even be, 

 in some sponges, contraction of the whole body; and 

 the early naturalists noted that these curious growths 

 did "sometimes seem to shrink from the hand that 

 tried to seize them." For the most part, though, the 

 ceaseless inner turmoil of the steadily pumping 

 sponge is masked by the deceptively quiet exterior. 

 And to the casual observer sponges are as unrespon- 

 sive as the rocks to which they grow firmly affixed. 

 During various periods in the past, sponges were 

 classed as plants, as plant-animals, and even as non- 

 living secretions produced by the great variety of 

 animals that take shelter in the numerous cavities of 

 a sponge. Not until the middle of the nineteenth cen- 

 tury were sponges finally assured of an unquestioned 



Living horny sponge, Hippospongia, sliced in half on being brought up from the bottom off 

 Batabano, Cuba. This is a commercial species; after the living tissue has been removed the 

 horny skeleton is used for washing walls and automobiles. { Ralph Buchsbaum ) 



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