Chap. 22 SPONGES — a side line of evolution 457 



out through the excurrent pore taking with it the various by-products of 

 metabolism. Each collared cell is a provider of food. Its flagellum brings in 

 the water that carries food; it captures and ingests the particles that the cur- 

 rent throws against it, and it partially digests them before they are passed on 

 to the ameboid cells that complete the process. Particles of solid waste are 

 eliminated from the various cells that perform the digestion. Each cell comes 

 very close to carrying on the whole process of nutrition essentially as it is in 

 the ameba, only a few degrees more specialized. 



The outer cellular covering of the body and the lining of the spongocoel are 

 separated by a layer of clear jellied secretion, the mesenchyme (Fig. 22.3). It 

 contains the versatile ameboid cells that move about easily in the yielding jelly. 

 When in contact with the collared cells, certain of the ameboid ones receive 

 food particles from them and complete the digestion of these. Certain others 

 secrete the crystal clear spicules of calcium carbonate; others are often packed 

 with excretory inclusions and pigment granules; still other cells are filled with 

 food and evidently act as storage reserves. 



Skeleton. Many people know sponges only as skeletons because the natural 

 sponges in general use are cleaned and bleached skeletons (Figs. 22.4, 22.5). 

 The skeleton is produced in the mesenchyme. It determines the shape of the 

 sponge, holds the water canals open, and is the support of the body. It is doubt- 



Ascon 



TYPES OF SPONGES 



Leucon 

 Complex like bath sponge 



Fig. 22.3. Body plans of three types of sponges: A, simple sponge; B, sponge 

 with folded wall; C, complex structure, e.g., bath sponge. Arrows denote currents 

 of water; short lines indicate flagella of the collared cells that line the food cham- 

 bers. 



