CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY OF 
THE MUSEUM OF COMPARATIVE ZOÓLOGY AT HARVARD 
COLLEGE. E. L. MARK, Drrecror. No. 125. 
A CASE OF INCOMPLETE DUPLICATION OF 
PARTS AND APPARENT REGULATION 
IN NEREIS VIRENS SARS. 
C. W.: PRENTISS. 
ALTHOUGH during the past few years numerous instances of 
complete bifurcation have been recorded in various annelids, 
I know of no case where intermediate conditions, or incomplete 
duplication of parts, have been observed. Among several hun- 
dred specimens of Nereis collected for class dissection in 
November, 1899, an individual was discovered apparently exhib- 
iting an abnormality of this kind, and it was therefore deemed 
worthy of special study and a brief description. 
The specimen we are to describe (Fig. 1), a sexually mature 
male, was collected at the mouth of the Saugus River near Lynn, 
Fic. — Vi 1 Nereis, l i I i seca 
ii) derit rome ans ., left n bey eo rg arma pa 
um ; so.’ r, first abnormal ideni v" ' 23, last abnormal metamere ; ior limit 
podi 
of the abnormal ventral region. 
Mass. It measured 11.5 cm. in length when fully extended, and, 
except for the region near the middle of its body, was perfectly 
formed externally. The variation from the normal was, un- 
fortunately, not observed until after the worm had been killed 
563 
