THE LEECHES OF MINNESOTA 43 



length of the commissure. Lavinow, '03, finds this same double 

 condition of the intercommissural cells in Protoclepsis tessellata, 

 and thinks there is some relation between t'he doubling of these 

 cells and the binucleate condition of the muscle cells. As nearly all 

 the muscle cells in P. pediculata are also binucleate, the same rela- 

 tion would seem to exist here. Appearances would seem to indicate 

 that amitotic division is the prevailing method for all these nuclei. 

 Lavinow, '92, would make the binucleate condition of the muscle 

 cells and the presence of two intercommissural cells, characteristic 

 for a new genus, Protoclepsis, which would separate it from Hem- 

 iclcpsis, on the one hand, and the Glossiphouidae (Glossiphonia, 

 Placobdella and Haementeria) on the other. The possession of both 

 of these characteristics by at least two species of Placobdella, viz. 

 P. pediculata, and P. parasitica, would indicate that there is a much 

 closer relationship between the genera Protoclepsis and Placobdella 

 than Lavinow imagined. 



The Anterior Ganglionic Mass. 

 (PI. E, Figs. 17, 18 and 19.) 



In P. pediculata there are no sharply defined supra- and sub- 

 oesophageal ganglia. The cell packets belonging to the suboesoph- 

 ageal ganglion of such leeches as C. hollensis Whitman extend in 

 this species far around toward the dorsal side of the oesophagus, 

 while the packets of the supra-oesophageal ganglion extend ventrad 

 beyond the median line, so that some packets belonging to the 

 former are much farther dorsal than some packets belonging to 

 the latter. The anterior ganglionic mass consists, as Whitman, 

 '92, has shown for C. hollensis, of six closely joined neuromeres, 

 each having the parts equivalent to one of the ganglia of the ventral 

 chain. The commissures are here shortened almost to complete dis- 

 appearance, leaving the ganglia so closely approximated that only 

 a small canal, across which runs the central Nerve of Faivre, re- 

 mains between the adjacent fibre masses. Above each packet is to 

 be found the usual central glia cell. 



The eight ventral packets of neuromeres III, IV, V, and VI are 

 arranged in a median ventral row, so closely crowded together that 

 each body of the row, with the exception of the two at the ends 

 which are about as long as wide, is two or three times as wide as 

 long. The ventral packets of somite II are somewhat smaller and 



