298 GENERAL ZOOLOGY 



that the Porifera, with their structure showing no sign of 

 digestive organs, muscle-cells, nerve-cells, or sense-organs, are 

 the simplest of the many-cell animals (Metazoa). Of course 

 we do not place the Porifera next above the Protozoa until 

 we have weighed the facts by comparing their structure with 

 the only other phylum that has two germ-layers. The Cce- 

 lentera have, as we recall, a gastrovascular cavity, simple 

 muscle-cells, nerve-cells, and sense-organs. 



An attempt to classify the animals that have three germ- 

 layers in development immediately brings us into difficulties. 

 Formerly it was the custom to place the Echinoderma near 

 the Ccelentera because of their radiate plan of structure, but 

 now the radial symmetry is not generally considered a factor 

 of great significance. The old group of Venues has been thor- 

 oughly studied in recent years, and, as indicated in Chapter 

 XVII, it has been subdivided into five phyla by some of the 

 best-known authorities. In the order of their increasing com- 

 plexity, the phyla are Platyhelminthes, Nemathelminthes, 

 Trochelmiiithes, Molluscoida, and Annulata. They are all 

 bilaterally symmetrical animals, but only the last phylum has 

 the body divided into somites. Because of this and other 

 facts, the order of classification is modified by the English 

 zoologists Parker and Haswell by placing the Echinoderma 

 between the Molluscoida and the Annulata. The Echino- 

 derma have an alimentary canal separated from the body- 

 cavity, digestive glands, a fairly well-defined blood-system, 

 an elaborate water-vascular system, and a definite nervous 

 system. For these reasons they are entitled to rank next to 

 the Annulata. All members of the Annulata have the body 

 divided into somites with unsegmented appendages. The 

 body has a distinct alimentary canal, a complete blood-system, 

 and a more complicated nervous system than is found in any 

 of the phyla already mentioned. The system of nephridia also 

 is more extended, one pair being present in every somite. 



