170 Practical Zoology. 



One form common on the northern Atlantic coast is a simple 

 or branched white tube, an inch or so in height, and sometimes 

 as thick as a pigeon's quill. These are in clusters, attached by 

 one end and open at the other. Embedded in the wall of each 

 tube are the spicules above mentioned, projecting both on the 

 outside and on the inside. The inside of the tube is lined with 

 cells bearing cilia which, by their vibration, drive the contained 

 water out of the mouth of the tube ; to replace which, water 

 enters through many holes which pierce the wall of the tube. In 

 sponges a little more complicated, the cilia, instead of lining 

 the main tube, are limited to small pouches, or lateral branches 

 of the main tube, extending into the body wall and communi- 

 cating with the exterior through small pores. In others the cilia 

 are found only in certain enlarged portions of these radiating 

 tubes. This represents the condition in the commercial sponges; 

 certain cavities are lined with cilia and are connected on the 

 one hand with the smaller tubes entering the whole surface of 

 the sponge, and on the other with the large tubes opening at the 

 top. These cilia cause the currents above mentioned. Thus 

 the sponge gets both food and oxygen. 



Sponges (including, besides those already mentioned, siliceous 

 sponges, whose spicules are flinty) constitute the branch Porifera. 



Read Commercial and Other Sponges, Hyatt. 



Topics for Reports. Sponge Fisheries. Experiences of 

 Divers. Sources of our Sponges. Fresh-water Sponges. 



