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 high. 



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, — e.g. 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, Tubularia. 



