PHYLUM H. 



CCELENTERATA. 



The coelenterates are radially symmetrical animals which possess 

 but a single internal cavity and no ccelom (Fig. 131). This cavity is a 

 simple space in a more or less cylindrical body in the lowest coelenter- 

 ates, but in the larger ones it is often extensively branched. The body 

 wall is composed of three layers, (1) an outer cellular layer, the ectoderm 

 (Fig. 131), (2) an inner cellular layer, the entoderm, and (3.) a tissue 

 between them called the middle or supporting layer which is skeletal in 

 function. 



This middle tissue in all ccelenterates but the CtenopJiora is pri- 

 marily non-cellular, being a secretion of the cellular layers, and is called 



the mesoglea; in the simplest 

 cases (Hydrozoa] (Fig. 131, A) 

 it remains non-cellular, but in the 

 larger and more complex forms 

 (Fig. 131, B) it becomes cellular 

 through the migration of cells 

 into it from the ectoderm or 

 entoderm. In the Ctenophora the 

 middle layer is primarily cellular, 

 being a development of the em- 

 bryonic mesenchyme. 

 The coelenterates are the lowest many-celled animals and are with- 

 out most of the organs and tissues which characterize the highest ani- 

 mals. Sexuality is, however, fully developed in all of them, some being 

 hermaphroditic, but the majority being unisexual. Asexual reproduction 

 by fission or budding is also very general and leads to the formation of 

 extensive colonies. Very many exhibit the phenomenon of alternation 

 of generation, in which a sexual, often free-swimming generation alter- 

 nates with a sessile, usually colonial and asexual generation. 



History. This phylum was constituted in 1847 by R. Leuckart, 

 who separated the polyps, medusae and Ctenophora (Eschscholtz) from 

 the Zoophyta-Eadiata of Cuvier and his contemporaries and called them 

 the Ccelenterata. He showed that these animals should be included in 



70 



Fig. 131 Diagram of a coelenterate, 

 with (A) a non-cellular and (B) a cellular 

 mesoglea. 1, ectoderm ; 2, entoderm ; 3, 

 supporting layer. 



