CHAPTER II. 



GENERAL STRUCTURE OP ZOOPHYTES. 



3. THE term zoophyte is applied to the whole animal structure, 

 whether a single animal, or consisting of a large number of animals, 

 as in most corals ; while the several individuals are called polyps. 



In view of their general radiated structure, the arrangement of 

 the tentacles around a disk as a centre, and a corresponding circular 

 structure within,* these animals are placed in the Fourth grand 

 division of the Animal Kingdom the RADIATA. The distinguishing 

 characteristics of the simple polyp, are as follows : 



An inarticulate fleshy body, nearly cylindrical, having a circular or 

 elliptical summit called the disk, bordered by one or more series of ten- 

 tacles, and an opening or mouth at the centre of the disk ; internally, a 

 visceral cavity closed below, no distinct vascular system, an imperfect 

 nervous system or none, and no senses but those of taste and touch. The 

 body, and, in most instances, the tentacles also are expanded by means 

 of water, which is ejected on contraction. 



Polyps are thus among the simplest of animals, being even less 

 complex in structure than the minuter Rotifers. A simple visceral 

 cavity, and a single opening to it placed at the centre above, with 

 traces of a radiated structure around it, are the only essential points ; 

 for even the tentacles are sometimes wanting. They have no intes- 

 tine, no glands to aid in digestion, separate from the general walls of 

 the internal cavity, no system of vessels in any part for circulation, 

 an imperfect nervous system, if any, no distinction of sex, and no 

 senses but those of taste and touch, with the latter of which the former 

 may properly be included. Moreover they are mostly dependent on 

 the fluid in which they live for the means of expanding their tenta- 



* Some of these animals are represented in figures 1, 6, and 12 beyond, and numerous 

 species are given in the Atlas. 



