10 THE COMING OF MAN 



successful organism, characterized by tendencies even more 

 than by details of structure. 



We left our protozoa as little groups or colonies of cells, 

 all alike except that in a few cases, as in volvox, certain cells 

 have been set apart for reproduction. That much can be 

 realized at a not much higher stage is proved by the very in- 

 teresting group of sponges, at which we cannot afford to cast 

 even a glance. We pick up our thread of development with 

 the coelenterates. These include our common sea-anemones 

 and their near relations, the coral animals, which have con- 

 tributed so much to the upbuilding and extension of coast 

 and mainland; also the jelly-fishes. The group shows con- 

 siderable variety of structure and attainment. Most of them 

 are sessile polj^s; the jelly-fishes are usually floating rather 

 than actively swimming forms. The general plan of the group 

 is well shown in our common fresh water hydra, which has 

 preserved many primitive characters. 



The green hydra is a small vase-shaped or tubular animal 

 about a quarter of an inch in length; the brown hydra may be 

 twice as large. It differs from the rest of the hydroid polyps 

 in not being permanently attached but moving over the stone 

 or stick, or swimming or floating in the water from place to 

 place. This exceptional habit seems primitive. In its struc- 

 ture it reminds us of a lady's shopping-bag, covered with silk 

 and lined with velvet. The opening into the bag corresponds 

 to the mouth of hydra, the cavity is for digestion, the silk 

 and velvet materials to two layers of cells enclosing the diges- 

 tive cavity or archenteron. The mouth is surrounded by a 

 ring of tentacles armed with nettle-cells for stinging and par- 

 alyzing its prey. During the breeding season we find on the 

 outside of the animal a little swelling, a reproductive organ 

 producing eggs or spermatozoa. 



The fundamental structural characteristics of hydra, and 

 of all coelenterates, are a body composed of two layers of 

 cells, ectoderm and entoderm, surrounding a primordial di- 

 gestive cavity, the archenteron, which in jelly-fishes sends out 



