THE ANIMAL KINGDOM 397 



Example: Aurelia, a large jellyfish, common on the Atlantic 

 coast (Fig. 224). 

 CLASS III. ANTHOZOA. Sea anemones and corals. 

 The body is cylindrical and attached at the aboral end by the 

 foot or pedal disk. The mouth is oval, giving the animal a 

 biradial symmetry, and is surrounded by from six to several 

 hundred tentacles. An ectodermal gullet leads from the mouth 

 to the gastrovascular cavity, which is subdivided by six or 

 more longitudinal mesenteries composed of mesoglea and 

 endoderm. The ectoderm secretes, in corals, a skeleton of 

 calcium carbonate, and in other forms a hornlike substance 

 called ceratine. All are marine and dioecious. 



Examples: Metridium marginatum, a sea anemone (Fig. 225) ; 

 Epizoanthus amerieanus, a sea anemone often attached to a 

 hermit crab; Porites porites, a common West Indian coral. 



PHYLUM 4— CTENOPHORA 



These beautiful marine animals are sometimes included in the 

 Phylum Coelenterata, which they resemble very closely (Figs. 

 226 and 227). The biradially symmetrical body is almost trans- 

 parent and may be round, oval, or ribbonlike in outline. Its 

 outer surface is soft and bears eight longitudinal rows of combs 

 whose teeth are composed of transverse plates of fused cilia. 

 Ctenophore means "comb bearer." In many, branched retractile 

 tentacles arise from pits near the aboral pole (Fig. 227). They are 

 provided with adhesive cells, which are used in capturing prey. 

 The slit-shaped mouth opens into a widened stomach region, to 

 which is connected a system of tubes opening to the outside by 

 minute pores. This complicated gastrovascular space is lined 

 with endoderm. Between the latter and the ectoderm is a thick 

 jellylike mesenchyme, which differs from the mesoglea of coelen- 

 terates in that it represents a true third germ layer. It also 

 contains muscle fibers of mesodermal origin. The animals, 

 therefore, are triploblastic. 



Lying in a pit at the aboral pole is a statocyst. 



Ctenophores are hermaphroditic, and pass through a compli- 

 cated metamorphosis before reaching the adult stage. Paedo- 

 genesis, reproduction in the larval stage, occurs in some species. 

 CLASS I. TENTACULATA. Characterized by a pair of long 



tentacles or a pair of oral lobes which, however, may appear 



only in the larva. 



