IG4 THE MECHANICAL FUNCTIONS. 



minant. In the construction of zoophytes, nature 

 seems still to keep in view the models of vege- 

 table forms, the characters of which, while 

 effecting the transition from one kingdom to 

 the other, she continues to impress on her pro- 

 ductions. Zoophytes, both in their outward 

 form, and in the disposition of their internal 

 organs, preserve the symmetrical arrangement 

 round a common centre so generally 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 composing this class : and 

 hence they have obtained the title 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 T'tihipora 



* See pa-re 90. 



