27 G LARVAL FORMS. 



and the prse-oral ring, though at first single, becomes at a later period 

 divided into two. This form is further exceptional in that the ciliated 

 rings are persistent in the adult. 



The unimportance of the character of the rings in the polytrochal 

 forms is shewn by such facts as the absence of these rings in Terebella 

 iiebulosa and the presence of dorsal half rings in Terebella conchilega. 



The mesotrochal forms are the rarest of Chtetopod larvce, and 

 would seem to be confined to the Chsetopteridse. 



Their most striking character is the presence of one or two complete 

 ciliated rings which girth the body between the mouth and anus. The 

 whole body is further covered with short cilia. The anus has a distinct 

 dorsal situation, while on its ventral side there projects backwards a 

 peculiar papilla. 



The total absence of the typical pros-oral and of the peri-anal bands 

 separates the mesotrochal larva? very sharply from all the previous 

 types. 



A characteristic of many Chietopod larva? is the presence of a 

 bunch of cilia or a single flagellum at the apex of the prae-oral 

 lobe. The presence of such a structure is characteristic of the 

 larval forms of many other groups, Turbellarians, Nemertines, Mol- 

 luscs, etc. 



In the preceding section the mode of multiplication of the 

 segments has already been sufficiently described 1 . 



Apart from the formation of the segments 

 the larval metamorphosis consists in the atrophy 

 of the provisional ciliated rings and other pro- 

 visional organs, and in the acquirement of the 

 organs of the adult. 



The great variations in the nature of the 

 Chsetopod appendages render it impossible to 

 treat this part of the developmental history of 

 the Chsetopoda in a systematic way. 



The mode of development of the append- 

 ages is not constant, so that it is difficult to 

 draw conclusions as to the primitive form from 



FIG. 151. LAEVA OF which the existing types of appendages are 

 PHYLLODOCE FROM THE i 



VENTRAL SIDE. (From 



Alex. Agassiz.) In a large number of cases the primitive 



rudiments of the feet exhibit no indication of 



1 It has been insisted by Semper (No. 35 t s) that certain of the anterior segments, 

 belonging to what he regards as the head region in opposition to the trunk, become 

 interpolated between the trunk and the head. The general evidence, founded on ob- 

 servations of budding, which he brings forward, cannot be discussed here. But the 

 special instance which he cites (founded on Milne-Edwards's (No. 347) observations) of 

 the interpolation of the head segments, bearing the gills, in Terebella appears to me 

 quite unjustified from Milne-Edwards's own statements; and is clearly shewn to be 

 unfounded by the careful observations of Claparede on Ter. conchilega, where the seg- 

 ments in question are demonstrated to be present from the first. 



