THE LOWEE FORMS OP LIFE. 121 



extended from the tube (b), which contains the rest of its body and 

 the germs of the future race of polyps, and it is able to protrude 

 from or draw into the tube by the contraction or relaxation of a pecu- 

 liar musculo-membranous structure. Now, where the polyps have 

 built up their tubes to say about the point c (Fig. 172), they simul- 

 taneously throw out a lateral process of lime-secreting coenosarc, 

 which processes, uniting together and becoming calcined, constitute 

 the platform (d, Fig. 172). Having effected this object, one or more of 

 the germs of each polyp become attached to the platform, grow up, 

 and, when they have arrived at the same height as their now dead 

 parents, they, like them, throw out lateral processes, form platforms, 

 and increase and multiply. Each stage, therefore, represents a 

 generation of polyps, the uppermost only being alive. 



So much for the Alcyonarian polyps with their eight tentacles, 

 and, for the most part, " foot " or "base " secreted skeletons. 



We will now turn to the Zoantharia, the polyp of which has five 

 or six tentacles, or multiples of those numbers, usually six, and whose 

 skeleton is built up upon the " tissue " or " sclero-dermic " system. 



This division comprises the great number of Corals in the families 

 Madreporidse, Fungidse, Poritidee, Astraeidse, Milleporidae, &c., and 

 with which I shall make the reader acquainted by a short notice of 

 the figures in the frontispiece. 



Another passage of the "Alcyonarian" to what has been well 

 termed the " Lamellarian" Corals, is effected by the clove-like form 

 known as Caryophyllia (Figs. 168-171, frontispiece). 



In these Corals the single polyp lives at the extremity of the dead 

 stony mass, and is able to retract itself within the cup-like 

 extremity (Fig. 168). When the polyp dies, its successor forms a new 

 cup for itself over that of its dead parent, and then the calcareous 

 structure increases in length. Sometimes it is single, and at others 

 it assumes a dendritic or arborescent form (Fig. 171, frontispiece). 

 Sometimes the external surface is smooth (Fig. 168, frontispiece), 

 or pitted, as shown in the allied genus Pocillopores (Fig. 160, frontis- 

 piece). 



In other instances, each cell is marked by a rim or circular inden- 

 tation, with longitudinal raised striae between each, as is seen in the 

 well-known form (Fig. 171, frontispiece). In all cases, however, 

 the single ^large sea-anemone, like polyp, lives alone at the end, and 

 its cup-like skeleton is divided into partitions of six, twelve, twenty- 

 four, &c., so that when the living polyp is removed a calcareous 

 lamellated structure of this kind is seen. In Fungia (Fig. 163, 

 frontispiece), each polyp enjoying a separate existence only forms 

 a single structure, the well known mushroom-shaped Corals of our 

 collections. Although I have represented in the figure a group of 



