PEDAL LACERATION IN ACTINIANS. 97 



there is in these forms no indication of the origin of the ciliated 

 bands as an acquisition of new or different tissues, as McMurrich 

 has suggested in arguing for a dual origin for the parts of the 

 complex filament, but, on the other hand, the lateral lobes of the 

 complex filament arise by a very slight differentiation of the cells 

 already present in the filament from the time of its origin from 

 the stomodeal ectoderm. In Fig. 17, PI. III., the filaments of 

 the dorsal directive, the ventro-lateral and the ventral directive 

 mesenteries all show in their centers the beginning of the growth 

 which will give to the mesogleal layer its characteristic branched 

 appearance. Each of these branches will become the center 

 and support, of one lobe of the adult trefoil filament. In the 

 dorsal directive mesenteries especially, it may be noticed that 

 the tissues of the filament have extended out around the edges 

 of the mesentery so that the filament is nearly circular in outline. 

 Originally the extension of the stomodeal ectoderm first appear- 

 ance of the filament was in the form of a flat sheet (Fig. 12, 

 PL II.). The separation of this sheet of tissue into the three 

 lobes of the adult filament apparently takes place through an 

 unequal growth from three points in the nearly circular filament, 

 as shown in Fig. 18, PI. III. 



As may be seen by comparing Fig. 9 and Fig. 12, PI. II., 

 with the fully formed filament (e. g., the filaments of the dorso- 

 lateral mesenteries in Fig. 17, PI. III.) the histological changes 

 which have taken place in the transformation of the "embryonic" 

 tissue of the stomodeal ectoderm to those of the tri-lobed filament 

 have been in the nature of a separation of the cellular elements 

 of the original tissue so that one type of cells (ciliated) is cen- 

 tralized in the lateral lobes; while the other types (gland and nettle 

 cells) are concentrated in the median lobe. The apparent 

 change is greatest in the case of the median lobe, since the un- 

 differentiated stomodeal ectoderm is at the time of the first 

 appearance of the filaments poor in gland cells and nematocysts. 

 This last-mentioned fact is also true of the activity growing 

 ectoderm of the new part of the column wall and oral disc, and in 

 all parts of the body there is a marked increase in the actual as 

 well as in the proportionate number of both these types of cel- 

 lular elements after the normal body proportions have been 

 established. 



