PLUMES OF CEPHALODISCUS. 223 



those of tlie other three species. The torsion desci'ibed by 

 Mastermau does not occur, bub the grooves of the first and 

 second pairs of plumes, developed in such a manner that 

 they face ventrally towards the buccal shield, continue to 

 face the shield. The last two pairs of plumes are not deve- 

 loped between the first two pairs and the shield, but on the 

 dorsal side of them, i.e. on the side remote from the shield. 

 The grooves of these last plumes (fifth and sixth pairs) are 

 not directed towards the shield, but away from it. 



Although the four series of plumes of buds were prepared 

 for the purpose of deciding the points raised by Mastermau, 

 they serve to show a number of other interesting features, 

 such as (1) the difference of plume development in the buds 

 of the four species; (2) the relation which the last-formed 

 pair of plumes bear to the edge of the post-oral lamella; 

 and (o) the conversion, in the later stages of growth, of the 

 line of the bases of the collar-outgrowths (plumes and post- 

 oral lamella) from a circle to a double crescent, or two 

 incomplete ellipses. I have selected from each of the four 

 series a number of preparations to illustrate these points ; 

 they are reproduced in text-figures 2, 3, 4, 6, 7, and 9, and 

 are described under headings of the respective species. 



Methods. 



In the case of the youngest stages the whole bud was 

 mounted in diluted glycerine, and gold size was run round 

 the edge of the cover-glass to keep it firmly in position, and 

 to prevent the glycerine from accumulating dust. Most of 

 the buds were dissected, the shield being first removed by 

 tearing through its stalk by the aid of fine mounted needles, 

 and then the collar region, with plumes and post-oral lamella 

 (e.g. text-fig. 2, j), was removed by carefully manipulating 

 the needles between these parts and the ''body" of the bud. 

 The three parts, shield, collar region, and " body " with its 

 stalk, were then mounted ou the same slide in dilute glyce- 

 rine. 



