BRIEF REVIEW OF THE SYSTEM OF LIFE. 431 



of each animal communicate freely with the tube in the stem ; and in this they differ from 

 Bryozoans, whose groups have no tubular axis. The ancient GraptoUtes (some of which 

 are represented on page 510) are supposed to have been of this nature. Others secrete 

 calcareous corals of large size, and are called Millepores (because the minute cells from 

 which the animals protrude are like pinpunctures in size, and very numerous over the 

 surface of the coral). The Millepores are common in the West Indies and other coral seas. 

 The minute animals of a Millepore have nearly the form represented in Fig. 440, which 

 represents a species of another genus, called Syncoryne. 



There are hence stony corals made by Polyps, by Hydrozoans, and by Bryozoans ; and 

 others that are made by sea-plants, as explained beyond. 



2. Actinozoans, Anthozoans, or Polyps. — Fleshy animals, like a flower in form, 

 having above (Figs. 435, 436) a disk, with a mouth at center, and a margin of tentacles ; 

 internally, a radiated arrangement of fleshy muscular plates ; and living for the most part 

 attached by the base to some support. Ex., the Actinia, or Sea-Anemone, and the ani- 

 mals of ordinary corals. 



There are two groups of coral-making Polyps : — 



1. AcTiNOiDS (Zoantharia) (Figs. 435, 436), which make the ordinary corals. The 

 rays or tentacles of the Polyps are naked, that is, without a fringe of papillae. In the 

 Madreporaria, the number of tentacles is a multiple of 6 ; in the Cyathophylloids or Tetra- 

 coralla, a multiple of 4. 



The coral is secreted within the Polyps, and not outside as in the case of shells. It is 

 usually covered with radiate cells, each of which corresponds to a separate Polyp in the 

 group. The calcareous rays or septa are made in the spaces between the fleshy partitions 

 in the interior of the Polyp. The material is calcium carbonate (limestone) ; and it is 

 taken by the Polyp from the water in which it lives, or from the food it eats. 



2. Alcyonoids {Alcyonaria) (Fig. 437), or those of the Gorgoniaand Alcyonium corals. 

 The rays of the Polyps are 8 in number, and fringed. Fig. 437 represents a part of a branch 

 of a Gorgonia (Sea-Fan), with one of the Polyps expanded. The branch consists of a 

 horn-like axis and a fragile crust. The crust is partly calcareous, and consists of the com- 

 mon tissue (coenenchyma) by which the Polyps are united together ; the axis is secreted 

 by the inner surface of the crust. The precious coral used in jewelry comes from the 

 shores of Sicily and some other parts of the Mediterranean, and belongs to this Alcyonoid 

 division. It is related to the Gorgonias, but the axis is red and stony (calcareous) instead 

 of being horny ; and this stony axis is the coral so highly esteemed. A few species make 

 calcareous corals much like those of the Actinoids without any separate crust. 



7. Spongiozoans. 



1. The Sponges (Porifera) are mostly complex groups of animals, having internal 

 membranes composed of ciliated cells resembling the collared Flagellate Protozoans. Some 

 simple sponges are of one Zooid only. The groups secrete, excepting in one tribe, — the 

 gelatinous Sponges, or Halisarcoids, — a framework (1) of horny fibers, or (2) of horny 

 fibers set with siliceous spicules ; or (3) of siliceous spicules or fibers ; or (4) of calcar- 

 eous spicules or fibers. Of these 4 kinds, the first are the Corneous Sponges of com- 

 merce ; the second, the Corneo- siliceous, a harsh and more brittle kind; the third, the 

 Siliceous ; the fourth, the Calcareous or Calcispongice. 



Some of the forms of the spicules of the corneo-siliceous and siliceous sponge-spicules 

 are shown in Figs. 446-460, by Hinde. All these spicules were found by Hinde in powder 

 filling a single small cavity m flint from Norfolk, England. All are much enlarged. 



The Hexactmellid Sponges are siliceous and have the framework made up of spicules 

 with rays crossing at right angles, making it 6-rayed at the crossing ; they are mostly 

 from great depths ; Tetractinellids are 4-rayeu. But simple forms accompany the more 

 complex. The Sponges occur at all depths in the ocean and are very various in shape. 



