SURVEY OF INVERTEBRATES 69 



spicules of calcium carbonate, embedded in the tissue. The latter 

 consists of two chief layers of cells: an outer, or dermal epithe- 

 lium, and an inner, or gastral epithelium, separated by a jelly- 

 like material containing many amoeboid wandering cells. 



The gastral epithelium is of particular interest because it con- 

 sists chiefly of a layer of collar cells, each provided with a flagel- 

 lum, and resembling certain flagellated Protozoa from which, 

 indeed, it is probable that the Sponges have descended. It is the 

 constant lashing of the flagella that creates the current of water, 

 laden with food and oxygen, through the pores into the gastral 

 cavity, and on out through the osculum, bearing excretions. It is 

 also the collar cells that capture, engulf, and digest particles of food 

 in typical unicellular fashion (intracellular digestion) and pass the 

 products on to the other cells. Similarly, respiration and excretion 

 are carried on by the individual cells. 



Such is the essential plan of structure of a simple Sponge, but 

 there are more complex forms which, in general, can be derived 

 from this so-called ascon type by a thickening of the body wall, 

 and then either the restriction of the collar cells to canals em- 

 bedded in it in close proximity to the gastral cavity (sYCONtype), 

 or their segregation in tiny cavities more remotely situated in the 

 tissues but still in communication with the gastral cavity by long 

 canals (rhagon type). The sycon type is represented by Grantia, 

 and the rhagon type by the common Fresh-water Sponge, Spon- 

 gilla, and the Bath Sponge, Euspongia. (Fig. 36.) 



But one must not gain the idea that all Sponges are quite 

 similar in outward appearance. As a matter of fact some are no 

 larger than a pin head, others are six feet or more tall. Some are 

 branched, fan-shaped, or cup-shaped. Most are white or gray 

 though many contribute brilliant colors to the picture presented 

 by the sea floor. 



Among all this variety of Sponges, however, certain species, 

 chiefly of the genus Euspongia, stand out as of high economic 

 importance because their flexible, fibrous skeleton of spongin, 

 when cleaned and dried, is the familiar bath sponge of commerce. 

 These Sponges, long gathered by diving and dredging, are now 

 also farmed. Live Sponges are cut into very small pieces, wired to 

 cement plates and then sunk to favorable places for sponge growth. 

 After several years the pieces have become Sponges of sufficient 

 size for the crop to be marketable. 



