200 
EDWIN S. OOnDEICH. 
of sphincter (fig’s. 33 and 36). A shallow branchial chamber 
lined with epidermis is thus formed, leading from the external 
to the internal opening. It is in this chamber that the 
nephridium opens, at a place corresponding apparently to the 
point of junction of the ectoderm with the endoderm (fig. 41). 
The position of the nephridiopore can be seen in figs. 36 and 
30. When the atrinm becomes formed by the closing off of 
tlie space between the metapleural folds, with which the 
branchial cavities become merged, the pores open into the 
atrium. A ventral view of a stage where the atrium has just 
begun to be formed posteriorly shows one or two nephridia 
behind the last open gill-slit (fig. 35). Probably these 
nephridia belong to the posterior gill-slits, which have closed 
up (Willey, 15) ; they open now directly on the surface 
(fig. 30). 
The young’ nephridium is a flattened sac, without internal 
opening (figs. 36 and 39). From its inner end spring a large 
number of solenocytes; their tubes pierce its wall, and their 
flagella pass into the lumen of the sac. The majority of the 
solenocytes spread over the blood-vessel which runs along 
the future dorsal edge of the slits. The solenocytes of the 
first few slits scarcely extend beyond this limit; but, passing 
backwards to more posterior nephridia, we find that the 
solenocytes spread farther and farther up towards the dorsal 
aorta, the tubes lengthening out as the cells lie farther from 
the nephridial sac. At about the fifth or sixth nephridium 
some of the solenocytes actually reach the aorta (fig. 40). 
fl'he tubes in this case ina,y attain a really astonishing length, 
stretching right across the field of a yVth oil-immersion 
objective with oc. 8. 
Fig. 34 represents the posterior gill region of a living 
larva, in which the remarkable development of the .soleno- 
cytes is well shown. Here a group of the longest solenocytes, 
some twelve to eighteen in number, spread out over the aorta 
in a most beautifully regular fan-like arrangement in each 
segment. A section of this region is shown in fig. 31 ; the 
fan-like disposition is found in each segment to the hindmost 
