170 THE BOTTOM OF THE SEA. 



from the depth of about four miles, are principally 

 formed of these spiculae; they accumulate in thick 

 and far-extended beds, the importance of which in 

 course of time, relative to the surface-form of the 

 globe, must be very considerable. 



6. Polypi — Their general structure — Reproduction of Polypi — 

 Vegetative life of Polypi — The Polypier — Two great classes 

 of Polypi distinguished by the form of the Polypier — The 

 Tubipora musica. 



Most of the polypi live in colonies, and find a point 

 of support on the rocks. We say live, but more 

 strictly speaking they vegetate, their movements 

 being extremely limited. Their bodies become en- 

 crusted with calcareous matter, and life gradually 

 withdraws from the petrifying animal. Eggs aban- 

 doned to the sea at various periods of the year, or 

 buds developed on the polypus, are two methods 

 equally common by which they are propagated. 



Tlie body of the polypus is soft ; its form is that 

 of a hollow cylinder; at one of its extremities an 

 opening serves for the introduction of aliment into 

 the body of the animal, and for the expulsion of 

 matters which have not served for its nutrition. This 

 single opening of the body is surrounded with fleshy 

 appendices or tentacles, more or less numerous ; the 

 digestive apparatus, however, is not always marked 



