THE SPONGES AND THE TWO-LAYERED ANIMALS 



139 



walled sacs perforated with hundreds of 

 microscopic holes (incurrent pores) and 

 one large opening, the osculum (excurrent 

 canal) at the upper tip. The cavity of the 

 sac is called the spongocoel. Its wall is 

 made up of an outer epidermis or skin-like 

 layer of flat cells, and an inner continu- 

 ous layer of the flagellated collared cells 

 ( choanocytes ) . A third jelly-like layer, the 

 mesenchyme, lies between these two, and 

 in this layer several kinds of amoeba-like 

 cells, called amoebocytes, are present. This 

 layer also contains the skeleton formed by 

 spicules, which resemble crystals. In the 

 glass sponge the spicules consist of siHceous 

 material and in horny sponges, of fibers 

 and spongin. In most forms, however, they 

 are composed of calcium carbonate. A 

 combination of spicules and fibers also 

 occurs. 



Scattered among the ordinary epidermal 

 cefls are the tubular pore cells, each with 

 a central canal or pore. A pore cell, together 

 with "helpers" surrounding it, controls the 

 flow of water into the sponge. The vigorous 

 beating of flagella on the collared cells 

 lining the spongocoel causes water to move 

 in through the pores and out through the 

 osculum. This movement causes a constant 

 stream of water, heavfly laden with micro- 

 scopic organisms, to pass within reach of 

 the choanocytes. The manner of beating of 

 the flagella in the collared cells propels tiny 

 food particles to the cell body which en- 

 gulfs it, forming food vacuoles much like 

 amoeba. Any food not needed by the 

 choanocytes is passed to the amoebocytes 

 for further distribution. Waste products are 

 borne out through the osculum in this same 

 current of water. 



Sponges in general 



Sponges reproduce asexually by means of 

 internal and external buds, as well as sex- 

 ually by means of eggs and sperms. Buds 

 form on the outside of the sponge and 

 sometimes move away, but as often remain 

 a part of the parent sponge. During unfa- 



vorable conditions, as in drought or cold 

 winters, sponges develop internal buds, 

 called gemmules, which are merely masses 

 of cells with a hard outer covering. They 

 drop to the bottom of the stream or sea 

 during these adverse conditions and grow 

 into sponges the next season when circum- 

 stances are again favorable. 



Some sponges are monoecious (of one 

 household), that is, both sexes are present 

 in one animal; others are dioecious ( of two 

 households), the sexes being found in sep- 

 arate animals. There are no special sex 

 organs, and the sperms and eggs simply 

 develop from certain of the amoebocytes in 

 the middle layer or mesenchyme. Fertiliza- 

 tion occurs in situ (in place) and is fol- 

 lowed by rapid division until a blastula is 

 formed with manv flacrellated cells (future 

 choanocytes). The flagella are directed in- 

 ward toward the central cavity, the blasto- 

 coel. A hole appears later and the embryo 

 turns inside out, bringing the internal 

 flaeellated cells to the outer surface. In this 

 stage, called the amphiblastula, it leaves 

 the mother sponge through the osculum 

 and swims around for a few days, finally 

 settlins and attaching itself to a rock or 

 some other solid object where it grows into 

 a sponge ( Fig. 8-4 ) . The larger cells of the 

 larva overgrow the flarellated cells and com- 

 pletely surround them. The cell layers of 

 the sponge seem to be the reverse of those 

 found in higher forms and for that reason 

 are not comparable to them. 



Some very interesting cases of commen- 

 salism are found among the sponges. They 

 occasionally strike up a friendly relation- 

 ship with animals such as crustaceans, 

 worms, and mollusks. Many animals make 

 the channels of the sponges their home; 

 even fish as long as 5 inches have been 

 found swimming about inside of sponges. 

 Certain species of crabs enlist the aid of 

 the sponge in camouflage by placing small 

 pieces of the sponge on their backs until 

 they become attached. Thus the crab is 

 amply hidden from its would-be food and 



