326 



EVENINGS AT THE MICEOSCOPE. 



The movable pieces inclose a skeleton of calcareous 

 substance, glassy, colourless, and brittle, in which, ac- 

 cording to the plan I have already described, are ex- 

 cavated a multitude of oval cavities which form irregu- 

 lar rows ; a central line runs down each piece, that is 

 solid and free from cavities. Tliis calcareous skeleton 

 is encased in a gelatinous flesh, similar to and continu- 

 ous with that of the stalk. 



Tliis is the smallest kind, the head being about 

 j'g th of an inch in height. 



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 dif- 

 fer, mutually approach, and appear 

 to run into each other. Tliis is the 

 case with the present, and with the 

 form Avhicli I will now show you. 



P. tridens is much larger than 

 anv of the other forms, the movable 



head being about ^^-th of an inch in 

 length, and the whole organ about 

 l-th ot an inch. This may be con- 

 sidered as essentially P. ti^ijjhylla^ 

 modified by the blades being greatly 

 drawn out in length, and at the same 

 time rendered quite slender, so that 

 they may be called pins ; they meet 

 only at the points, where they often 

 cross, the interspaces of the basal 

 The inner edges of these are notched 

 with teeth as in P. triphylla, of which tliose near the 



HEAD OF PEDTCELLAKIA 

 TEIDKNS. 



parts being open. 



