MOSS'LIKE AND JELLY-LIKE ANIMALS 



Chapter 3 



Under the heading of moss-Hke and jellyHke animals are grouped some 

 forms which our modern system of classification separates into three distinct 

 phyla and which, in spite of their superficial likenesses, are really distant rela' 

 tives. Until modern microscopes revealed the details of their structures, all of 

 these creatures were placed, even by the scientist, as zoophytes or plant' 

 animals. 



The first of these groups is the Porifera or Sponges, most of which are 

 marine, but which have several small fresh'water forms. None of these much 

 resembles the skeleton of the marine sponge, which we use for washing pur- 

 poses, but may be truly called moss'like. They form mats or irregular masses 

 on the under sides of floating logs and boards and sometimes are found as 

 patches around the submerged stems of rushes. The resemblance to mosses 

 is still further increased in some of the common species by the green color, 

 which is probably due to associated or symbiotic algae. Sponges are usually 

 distinctive, however, in having a characteristic odor, which has been described 

 by some writers as resembling that of garlic. Occasionally under favorable 

 conditions sponges become very numerous and have been known to grow in 

 and even fill the pipes from reservoirs. 



The shape of the sponge colony varies and the only reasonably constant 

 character upon which identification can be based is the form of the silicious 

 needles or spicules which are embedded in and support the body wall. There 

 are usually two kinds of spicules in the body wall : large skeletal spicules, which 

 are often bound together in bunches, and smaller flesh spicules, which are scat' 

 tered through the body mass. Spicules of a somewhat different character are 

 developed around the gemmules or small masses of cells by means of which 

 the sponge reproduces itself. These gemmules arise throughout the body and 

 appear as small, round, dark objects in the sponge colony. They become most 

 numerous in the autumn and live through the winter, after the colony has 

 disintegrated. Autumn is the best time to collect sponges, since gemmules are 

 usually necessary for positive identification. Sponges may be merely dried 

 out, when gathered. Later the collector can crush a portion in a drop of water 

 for examination and identification under the microscope at his leisure. Hot 

 nitric acid is often used to dissolve the sponge mass, after Vv'hich the spicules 

 are washed ofi^ and examined more easily. 



The next group in this series is called the Coelenterata, the fresh-water 

 forms of which are mainly plant-like. Back in 1744 a man named Trembley 



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