. An introduction to zoology, with directions for practical work (invertebrates). 48 INTRODUCTION TO ZOOLOGY Coral reefs always tend to spread outwards, away from the land, for the polyps flourish best where they are most exposed to the splash of the waves which bring them air and food. All the common reef- building corals are similar to Madrepora in having six—or some multiple of six—tentacles, and a skeleton which extends into the body-wall of each polyp. The Red The Common Coral. Eed Coral {Goral- lium ittbrum), which is used for ornaments, belongs to a separate division of the Antho- zoa,
. An introduction to zoology, with directions for practical work (invertebrates). 48 INTRODUCTION TO ZOOLOGY Coral reefs always tend to spread outwards, away from the land, for the polyps flourish best where they are most exposed to the splash of the waves which bring them air and food. All the common reef- building corals are similar to Madrepora in having six—or some multiple of six—tentacles, and a skeleton which extends into the body-wall of each polyp. The Red The Common Coral. Eed Coral {Goral- lium ittbrum), which is used for ornaments, belongs to a separate division of the Antho- zoa, with polyps having eight tentacles and eight mesenteries. The common stem of the col- ony in this form, secretes an internal axial skeleton, which is calcareous, solid, and inflex- ible, the polyps projecting like buds from the enveloping living cells. This precious coral is obtained chiefly from the Mediterranean Sea, on the floor of which it forms branching masses about one foot Fig. 24. —GoraUium rubntm. (After Laoaze-Duthiers.) Classification of the Ooelenterata mentioned in Chapters II. and III. Class I. Hydrozoa.—Here the dominant phase is a Hydra- like form, either solitary or forming a branched colony. In the colonial forms, special individuals are usually modified for repro- duction, and in some cases these become transformed into free- swimming medusae; they may, therefore, be said to develop as lateral buds from the colony. The sexual cells are always discharged directly into the surrounding water. Sub-class A—Sea Firs, those with horny cups to protect the polyps, — Sertularia, Antennularia, Plumularia, Cam- panularia, Obelia. Sub-class B.—Zoophytes, those in which the mouth-cone and tentacles of the polyp are not protected by a skeletal cup,— Bougainvillea, Cordylophora, Hydra, Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance
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