322 EVENINGS AT THE MICROSCOPE 



Considerable modifications are found to exist in the 

 details 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 -g^th of an 

 inch in length, and the whole organ 

 about -Jth of an inch. This may be con- 

 sidered as essentially P. triphylla, modi- 

 fied 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 interspaces 

 of 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 cat into subordinate 

 teeth of exquisite minuteness. 



We have here an opportunity of see- 

 ing that the oval or square markings, 



. HEAD OF PEDICELLARIA 



TRIDENS. which are thickly placed throughout 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 edge of the fracture is not 

 even, but jagged with holes exactly corresponding with 

 the marks in question; so that the structure is the same as 

 that of the spines and of all the other solid parts of the 

 Urchin. 



