398 WILSON. [Vol. VI. 



gives several figures of the larva of Pomatoceros, in which the 

 numbers of the prototrochal cells are respectively 19, 18, 17, 

 and 15, but he does not appear to have given special attention 

 to the point. In Eiipomatus Hatschek figures specimens with 

 10, 9, and 8 prototrochal cells, but makes no mention of their 

 number in the text. Kleinenberg states that in Lopadorhynchits 

 there are usually fourteen nuclei in the prototroch (the cell- 

 outlines could not be distinguished). The only form that I 

 have carefully examined, besides PolygoTcims and Nereis, is 

 Hydroides {Serpiild) diantJius, Verrill, in which the prototro- 

 chal cells of the young free-swimming trochophore are eight in 

 number. 



It is easy to suggest comparisons between the eight-celled 

 prototroch of Hydroides, the eight-celled girdle of Nereis, the 

 eight series of vibratile plates in the Ctenophore, etc., but such 

 comparisons have no value on account of the paucity of data. 

 It would, however, be very interesting to investigate the com- 

 parative development of the prototroch, for it is certain that 

 the cells vary in number in different annelids. If Kleinenberg 

 has correctly determined their number in Lopadorhynchiis, it 

 would seem that they must arise by a different law of cell- 

 division from that of Nereis, since the number of cells is not a 

 multiple either of three or of four. 



It will be convenient to proceed directly to a brief sketch of 

 the later history of the prototroch. The original girdle lies 

 considerably above the equator of the embryo, but the proto- 

 trochal girdle gradually passes downwards until it takes a nearly 

 equatorial position. The cells become somewhat elongated and 

 numerous vacuoles appear in them. About the tenth or eleventh 

 hour the cilia appear and the larva begins to rotate slowly (Fig. 

 60). As in the case of many other annelids, the cilia are put 

 forth through pores in the egg-membrane, which appears to 

 persist as the cuticle of the adult worm. The development of 

 the cilia is very sudden, the operation requiring only a few 

 minutes. At the same time the apical cilia are put forth in the 

 following curious manner. The egg-membrane is at this period 

 separated from the upper side of the embryo by a considerable 

 space. A narrow process is now rather suddenly put forth 

 from the middle of the upper hemisphere (Fig. 60). This 

 process is extended until it comes into contact with the mem- 



