154 ZOOLOGY. 



manner to that by which the Amoeba is fed. " The 

 sponge represents a kind of subaqueous city, where the 

 people are arranged about the streets and roads in such 

 a manner that each can easily appropiiate his food from 

 the water as it passes along." (Huxley.) 



222. Reproduction in the sponges is effected in two 

 different ways. In the fresh-water sponge (spongilla), 

 towards the approach of winter, extremely minute 

 bodies called seeds, or gemmules, are formed in 

 the inner layer. One of these gemmules consists of a 

 number of sponge particles, coated over with a cyst. This 

 cyst has imbedded in it spicula of a very curious form, 

 consisting of an axle with a toothed wheel at each end. 

 During the winter the spongilla dies ; and, on the arrival 

 of spring, the sponge particles pass out through an open- 

 ing in the cyst, and become sponges like the original. 



During the summer months some of the sponge par- 

 ticles are changed into ova, others into sperm cells. 

 The resulting embryos are provided with cilia. After 

 swimming about for a time they lose their cilia, attach 

 themselves to some solid object, and are developed into 

 sponges. 



223. Distribution. Sponges are found both in salt 

 and fresh water, and are met with in most parts of the 

 world. The sponges of commerce are all marine, and are 

 confined to the warmer regions. The best sponges are 

 obtained from the Greek islands, a coarser kind being 

 procured from the Bahamas. Fossil sponges are common 

 in various geological formations, but are especially 

 abundant in the chalk. The flint nodules which abound 

 in this formation are believed to have been mainly formed 

 by them. It is an interesting and suggestive fact that 

 siliceous sponges, similar to those found in the chalk, have 

 been recently dredged from the bottom of the Atlantic, 

 along with the shells of the Foraminifera. 



