272 PRINCIPLES OF ANIMAL BIOLOGY 



Phylum PORIFERA. Diploliliislie, radially symmetrical animals with body wall 

 penetrated by numerous pores. Body usually supported by a skeleton of spicules or 

 spongin. Sponges. 



Class CALCAREA. Sponges with spicules composed of calcium carbonate, mon- 

 axon or tetraxon in form. (Figs. 127, 128.) 



Class HEXACTINELLIDA. Sponges with spicules composed of siUcon, triaxon 

 in form. 



Class DEMOSPONGL^. Sponges with spicules composed of silicon, not triaxon 

 in form, or skeleton composed of spongin, or with skeleton of both spicules and spongin. 



Phylum CCELENTERATA. Diploblastic, radially symmetrical animals with ten- 

 tacles, stinging cells, single gastrovascular cavity, no anus. Two body forms are 

 prevalent, the hydroid and the medvisa. Jellyfishes, polyps and corals. 



Class HYDROZOA. Ccelenterates without stomodaeum and mesenteries; sexual 

 cells discharged to the exterior ; hydroid and medusa forms in the life history of same 

 species, or only the medusa, the latter having a velum. Polyps (including Hydra), a 

 few corals, small jellyfishes. (Figs. 60-65, 212, 216.) 



Class SCYPHOZOA. Ccelenterates with only the medusoid, not hydroid form; 

 velum lacking ; notches at margin of umbrella. Larger jellyfishes. 



Class ANTHOZOA. Ccelenterates without medusoid forms, with well developed 

 stomodaeum and mesenteries. Sea anemones, most corals. Fig. 71, part. 



Phylum CTENOPHORA. Triploblastic animals; symmetry partly radial, partly 

 bilateral ; eight rows of vibratile plates radially arranged. Sea walnuts or comb jellies. 



Phylum PLATYHELMINTHES. Triploblastic, bilaterally symmetrical animals 

 with body flattened, with a single gastrovascular cavity (sometimes wanting) and no 

 anus. Flatworms. 



Class TURBELLARIA. Free-living flatworms with ciliated epidermis. Planaria. 

 (Figs. 81, 82.) 



Class TREMATODA. Parasitic flatworms without cilia but with a hardened 

 ectoderm, usually parasitic and with attaching suckers. Flukes. (Fig. 140.) 



Class CESTODA. Parasitic flatworms with the body differentiated into a head 

 (scolex) and a chain of similar joints (proglottides), the whole being usually regarded 

 as a colony. Tapeworms. (Figs. 68, 96.) 



Phylum NEMATHELMINTHES. Bilaterally symmetrical, triploblastic animals 

 with an elongated cylindrical body covered with a cuticle, with a 'true body cavity, 

 and a digestive tract with both mouth and anus. Roundworms. 



Phylum ECHINODERMATA. Radially symmetrical (with minor exceptions), 

 triploblastic animals with well developed ceelom, and usually with five antimeres, 

 spiny skeleton of calcareous plates, and organs of locomotion known as "tube feet" 

 operated by a water-vascular system. Starfishes, sea urchins, sea cucumbers. 



Class ASTEROIDEA. Free-Hving, typically pentamerous echinoderms with wide 

 arms not sharply marked off from disc and with ambulacral grooves. Starfishes. 



Class OPHIUROIDEA. Free-Uving, typically pentamerous echinoderms with 

 slender arms sharply marked off from disc and no ambulacral grooves. Brittle stars. 



Class ECHINOIDEA. Free-living, pentamerous echinoderms without arms; test 

 composed of calcareous plates bearing movable spines. Sea urchins, sand dollars. 



Class HOLOTHURIOIDEA. Free-living, elongated, soft-bodied echinoderms 

 with muscular body wall and tentacles around mouth. Sea cucumljcrs. 



Class CRINOIDEA. Sessile echinoderms with five arms generally branched with 

 pinnules, aboral pole usually with cirri, sometimes with jointed stalk for attachment 

 to substratum. Feather stars, sea lilies. 



Phylum ANNELIDA. Triploblastic, bilaterally symmetrical elongated animals 

 with external and internal segmentation; coolom usually present; setaj common. 

 True worms. (Figs. 67, 83, 95, 142, 143, 144.) 



