STIUHITURE OK THE EXCHETORY OROANS OF AMI’HIOXUS. 197 
The nephi’idium of Hatschek reaches its maximum develop- 
ment in the adult, where it is indeed the largest nephridium 
in the body, some 2 mm. in length. Lying on the left side, 
below and parallel to the notochord, it opens just behind the 
velum into the pharynx,^ and runs forward a long distance 
to a point just in front of the ciliated groove (Riiderorgan). 
Here it ends blindly, and along its course are given off short 
blind diverticula (figs. 27, 42, 43, 44). Solenocytes are set 
on the dorsal and lateral surfaces of the organ along almost 
its whole length, being especially numerous on the diverti- 
cula (fig. 28). Altogether an enormous number of soleno- 
cytes are present on this nephridium in the adult Am- 
phioxus. 
The canal runs along the floor of a narrow cavity beside 
the aorta (figs. 42 — 44). It is to the wall of this cavity that 
the solenocytes are attached, and it appears to be of coclomic 
nature; at all events it is in open communication with the 
myococle of the first myotome in larval stages (figs. 25, 20, 
33). In the adult, however, it is closed off, and the lining 
epithelium seems to be very irregularly developed, forming 
Jio distinct layer of cells (fig. 28). 
In the larva of about 13 gill-.slits, of the left series only 
(fig. 33), the nephridium cau be well seen by transparency 
as a short tube opening behind into the phai’ynx (fig. 24). 
Its dorsal surface is entirely beset with solenocytes in several 
closely packed rows (fig. 29). An optical section of the 
organ at this stage is represented in fig. 38, showing clearly 
the way in which the tubes of the solenocytes pierce the thin 
dorsal wall. 
We may summarise as follows the observations recorded 
above : — The nephridium of Hatschek is a true nephridium, 
similar in structure to the posterior paired nephridia. In 
the adult, where it reaches its maximum development, it 
extends along the left aorta from in front of the ciliated 
‘ On one occasion only I have found an opening from the canal into 
the hinder region of the huccal cavity itself, as well as the posterior 
opening into the pharynx. 
