284 EVENINGS AT THE MICROSCOPE. 



from cavities. This calcareous skeleton is encased in a 

 gelatinous flesh, similar to and continuous with that of the 

 stalk. 



This is the smallest kind, the head being about 3 yh of 

 an inch in height. 



Considerable modifications are found to exist in the de- 

 tails of each form, in the relative proportions which the 

 parts bear to each other, and so forth ; so that two forms, 

 which in their extreme conditions widely differ, mutually 

 approach, and appear to run into each other. This is the 

 case with the present, and with the form which I will now 

 show you. 



P. tridens is much larger than any of the other forms, 

 the movable head being about ^th 

 of an inch in length, and the whole 

 organ about th of an inch. This 

 may be considered as essentially 

 P. triphylla, modified by the 

 blades being greatly drawn out in 

 length, and at the same time ren- 

 dered quite slender, so that they may 

 be called pins ; they meet only at 

 the points, where they often cross 

 the spaces between the basal parts 

 being open. The inner edges of 

 these are notched with teeth as in 

 P. triphylla, of which those near 

 the tips are larger and cut into sub- 

 ordinate teeth of exquisite minute - 



We have here an opportunity of 

 seeing that the oval or square mark- 

 Z 8 > which G thickly placed through- 

 ou t the calcareous substance of the 

 blades, are certainly cavities in it ; for in those examples 

 in which the pins, which are very brittle, are broken, the 



