124 THE MECHANICAL FUNCTIONS. 



of the animal kingdom retain with plants, are more marked 

 and more predominant. In the construction of zoophytes, 

 nature seems still to keep in view the models of vegetable 

 forms, the characters of which, while effecting the transition 

 from one kingdom to the other, she continues to impress on 

 her productions. Zoophytes, both in their outward form, 

 and in the disposition of their internal organs, preserve the 

 symmetrical arrangement round a common centre so gene- 

 rally exhibited in plants, and especially in flowers, and in 

 the verticillated leaves and branches.* Hence, the radiated 

 or star-like forms which predominate in most of the animals 

 cbmposing this class: and, hence, they have obtained the ti- 

 tle of Radiata, by which Cuvier has designated them. 



Like the animals of the sponge tribe. Polypi are for the 

 most part attached to some inorganic shell or base, which 

 may be either of a horny or calcareous nature. The form 

 of this shell admits of almost infinite variety. In some it 

 constitutes the external surface of the animal, and encloses 

 the flesh in a general sheath, leaving only openings at the 

 extremities of the tubes for the expansion of each set of 

 tentacula surrounding the respective mouths. Sometimes 

 these tubes are placed parallel to each other, like the pipes 

 of an organ, with transverse partitions at regular intervals: 

 such is the structure of the Tubipora miisica, as shown in 



Fig. 61. In Fig. 62, a portion of the 

 tubes is seen highly magnified, and laid 

 open, to show the polypes in their inte- 

 rior. At other times the tubes are joined 

 together endwise, like the branches of a 

 tree, leaving lateral apertures for the pro- 

 trusion of the tentacula of each separate 

 polype: this is the case in the Seriularia. 

 (Fig. 60.) 



In some species the horny base is fashioned into a num- 

 ber of cells, each of which serves for the protection of its 

 respective polype. These cells are generally placed at the 



* See page 7Q. 



