504 FRANK E. BEDDARD. 
below the alimentary tract is the ventral blood-vessel; on the 
opposite side is the brain (dr.). The stomodzum is easily 
recognisable from the mesenteron by the different character of 
its epithelium ; as far as I could ascertain the lumen of the 
stomodzeum was not continuous with that of the mesenteron. 
Whether the anterior two segments contain provisional 
nephridia or not, I am unable to say; this is, unfortunately, 
another of the many questions in the embryology of Acantho- 
drilus for the solution of which I do not possess material. I 
am inclined, however, to think, as will be pointed out later, 
that the nephridia of the first two segments have no pre- 
cursors. 
Stage B. 
The size of embryos belonging to this stage is indicated in 
fig. 20 s. I have only had three embryos for study, one of 
which was slightly larger than the others, but not in a more 
advaneed condition of development. 
These embryos are completely vermiform in shape, but the 
anterior end is still considerably the thicker, and dwindles 
gradually towards the tail end. 
Fig. 22 represents a section through the first four segments 
of an embryo belonging tv this stage. The sete are numbered 
consecutively, the first seta (J) corresponding, of course, to 
the 2nd segment. ‘The first pair of nephridia seems to occupy 
three segments, but the irregular arrangement of the muscular 
bands attaching the pharynx (stomodzum) to the parietes make 
it a little difficult to be very certain upon the point. Both 
the funnel (f.) and the external pore (N. p.) were quite obvious 
at this stage; the funnel lies, as before, in the 2nd or 38rd seg- 
ment. The external orifice of the nephridium is just within the 
stomodeum. ‘There is, in fact, no particular change from the 
conditions characterising the last stage. 
Stage C.’ 
Embryos of this stage measure rather more than half an 
inch in length; they are, of course, completely vermiform in 
shape. 
