292 - Multicellular Animals, Especially Man 



the osculum, a large opening at the upper 

 extremity, and also there are many small COLLAR 

 pores in the side walls of the sponge. The CELL 

 entire tube is lined internally with flagel- 

 lated cells (Fig. 16-2). These set up a stream 

 of water that enters through the pores and 

 leaves through the osculum (Fig. 16-1). Some 

 of the cells, which are aggregated around pit- 

 like chambers, possess collars encircling the 

 flagella (Fig. 16-2). The (lagella of these cells 

 serve to waft small particles of food into the 

 collars, whence the food passes into gastric 

 vacuoles in the individual cells. Here the col- 

 loidal foods are digested. Part of the glucose, 

 amino acids, and other products of digestion 

 are utilized by the cells in which they are t acle is armed with numerous evenly ar- 

 liberated, but part diffuses to the neighbor- rang ed batteries of stinging cells (Fig. 16-4). 

 ing cells. No parts of the body are very re- When the stinging cells discharge, they inject 



Fig. 16-2. Section of o flagellated chamber of a 

 sponge. 



mote from the source of absorption. The wall 

 of the sponge is composed of only two cell 

 layers, although some sponges have a large 

 amount of relatively inert material, the 

 mesoglea, interposed between the outer and 

 inner cell layers. In commercially valuable 

 sponges (Fig. 16-3), the mesoglea is composed 

 of a silklike protein substance (spongen); 

 but in others the skeletal material is calcare- 

 ous or silicaceous, according to the species. 



THE SACCULAR DIGESTIVE SYSTEMS 

 OF HYDRA AND PLANARIA 



In simplest form, the saccular type of di- 

 gestive cavity is found in the fresh water 

 polyp, Hydra. Essentially this small familiar 

 aquarium animal has the form of a sac, in 

 which one opening, the mouth, communi- 

 cates with the surrounding water (Fig. 16-4). 

 The body wall of Hydra is composed of only 

 two cell layers; but between the outer ecto- 

 dermal epithelium and inner endodermal 

 epithelium, there is a thin layer of nonliving 

 matrix, the mesoglea. 



Hydra captures a variety of small free- 

 swimming animals by means of tentacles, a 

 group of slender, mobile, threadlike organs 

 that originate from the body in the region 

 surrounding the mouth (Fig. 3-12). Each ten- 



a paralyzing fluid into such victims as may- 



Fig. 16-3. Yellow sponge. The skeletal material of 

 this sponge is spongin, a silklike compound. (The ascu- 

 lum is at the top.) (Courtesy of the Fish and Wildlife 

 Service, U.S. Department of the Interior.) 



