APPENDIX B 



635 



sponges consist of indistinguishably fused individuals and may weigh hundreds 

 of pounds. 



About 3,000 species of sponges have been described. 



Phylum III. COELENTERATA (sel en' ter a' ta; Greek, koilos, "hol- 

 low," and enteron, "intestine"). 



The hydroids, jelly fishes, sea anemones, and corals (Fig. B.4). Simple 

 Metazoa, mostly marine. Exhibiting usually the following characters: 



Fig. B.4. Representative coelenterates. A and B, Hydrozoa. A, Petasus, a medusa or jelly- 

 fish. B, Halistemma, a colony of differentiated polyps related to the Portuguese man-of-war. 

 C to E, Scyphozoa; three types of medusae. F to «/, Anthozoa. F , Metridium, a sea anemone. 

 67, Madrepora, a coral. H, Corallium, the precious red coral of the Mediterranean. /, 

 Tubipora, the organ-pipe coral. J, Pennatula, one of the sea pens. {Modified from Turtox 

 chart, courtesy General Biological Supply House, Inc.) 



Body wall diploblastic, with an outer germ layer, the ectoderm, and 

 an inner, the entoderm, enclosing a third jellylike, noncellular layer, the 

 mesoglea, which may become very thick in some forms (jellyfish). Somatic 

 cell differentiation marked. Radial symmetry, modified by an increasing 

 tendency toward bilateral symmetry in the higher species of the phylum. 

 No metamerism. Possessed of a coelenteron — a single body cavity lined 

 with entoderm and opening to the outside by a mouth but lacking an 

 anus. The typical sac-type of body organization occurs throughout but is 

 modified to form either a tubular polyp or a bell- or umbrella-shaped 

 medusa (jellyfish). 



Among the features peculiar to this group is an alternation of generations, 

 called metagenesis, whereby a usually sessile and colonial asexual polyp generation 



