No. 415.] NEREIS VIRENS SARS. 567 
only two projections, both of which belong to the dorsal ramus ; 
the smaller is the dorsal cirrus, and the larger one the superior 
ligula of the dorsal ramus. Both acicula, as well as the ven- 
tral ramus, are wanting. In the third metamere posterior to 
this section, as we have seen from an examination of the exte- 
rior, the normal structure is once more regained. 
Throughout the abnormal region the musculature of the 
body wall and that of the appendages are well developed in the 
interpolated portion. In the body wall of the interpolated 
part circular, longitudinal, and oblique muscles are found. The 
circular band is continuous with that of the normal portion 
of each metamere. The supernumerary longitudinal muscles 
Fic. 4. s Dae ot the anterior face of a section through the twenty-first abnormal somite and 
its appendages. (X 6.) «cd.z., nerve cord; dx., right normal parapodium; Zx.', right 
abnormal parapodium ; #zz./., ongidina ae ee a n normal ibus rium 1 
5. , left abnormal parapodium ; va.v., ven blood ve: 
(Fig. 2, »4.//) have in cross-section the peculiar kidney shape 
characteristic of the normal longitudinal ventral muscles. They 
appear as small strands of fibres in the first abnormal meta- 
mere, and are attached to the circular muscle fibres of that 
region. They attain their greatest development in the next 
few somites, and after extending back through fifteen meta- 
meres are gradually reduced in size and finally disappear. Sets 
of oblique muscles are also found in those segments in which 
the extra, ventral longitudinal muscles are present. Parapodial 
muscles are attached to the proximal ends of the acicula, as is 
usual in normal appendages. 
The intestinal canal shows no particular irregularity of form. 
There is no sign in the abnormal metameres of even partial divi- 
Sion, but its size is remarkedly increased throughout this region. 
