GENERAL SKETCH OF THE POLYPS. 



FlK- 1. 



GENERAL SKETCH OF THE POLYPS. 



Before describing the different kinds of Polyps living on our 

 immediate coast, we will say a few words of Polyps in general 

 and of the mode in which the structural plan common to all 

 Radiates is adapted to this particular class. In all Polyps the 

 body consists of a sac divided by vertical partitions (Fig. 1.) into 

 distinct cavities or chambers. These parti- 

 tions are not, however, all formed at once, but 

 are usually limited to six at first, multiplying 

 indefinitely with the growth of the animal in 

 some kinds, while in others they never in- 

 crease beyond a certain definite number. In 

 the axis of the sac, thus divided, hangs a 

 smaller one, forming the digestive cavity, 

 and supported for its whole length by the six 

 primary partitions. The other partitions, though they extend 

 more or less inward in proportion to their age, do not unite 

 witli the digestive sac, but leave a free space in the centre be- 

 tween their inner edge and the outer wall of the digestive sac. 

 The genital organs are placed on the inner edges of the partitions, 

 thus hanging as it were at the door of the chambers, so that 

 when hatched, the eggs naturally drop into the main cavity of 

 the body, whence they pass into the second smaller sac through 

 an opening in its bottom or digestive cavity, and thence out 

 through the mouth into the water. In the lower Polyps, as in 

 our common Actinia for instance, these organs occur on all the 

 radiating partitions, while among the higher ones, the Ilalcy- 

 onoids for example, they are found only on a limited number. 

 This limitation in the repetition of identical parts is always found 

 to be connected with structural superiority. 



The upper margin of the body is fringed by hollow tentacles, 

 each of which opens into one of the chambers. All parts of the 

 animal thus communicate with each other, whatever is intro- 

 duced at the mouth circulating through the wliole structure, 



Fig. 1. Transverse section of an Actini.a. (Jijassii.) 



