258 E, RAY LANKESTER. 
unnatural contraction and shrinking. Secondly, the season 
at which the specimens are taken may be either that in which 
the generative products are undeveloped or that in which they 
are in full maturity, and accordingly very considerable differ- 
ences in the parts connected with the reproductive organs are 
to be observed. These causes will not, however, explain 
sucha statement as that of Stieda, namely, that the pharynx 
is not perforated by slits leading to the atrial chamber. ‘his 
is simply an error of observation. 
1. The Pharynz.—It is not difficult to satisfy one’s self, by 
the examination of sections and detached flakes from the pha- 
rynx, that it is really perforated by oblique-running clefts, 
and that these form a communication between it and the atrial 
chamber which opens to the exterior by means of the ab- 
dominal pore (atriopore). Stieda’s statements on this point 
appear to me to be quite erroneous. The atrial chamber (posto- 
ral atrium or epiccel), like the mouth cavity (preoral atrium), 
is lined with a brown pigmented membrane which is con- 
tinuous developmentally (in accordance with Kowalewsky’s 
observations as to the mode of development of the atrial 
chamber) with the epiblast in each case. Now, this brown 
pigmented membrane serves as a clear and simple line of demar- 
cation by which to trace out the limits of the atrial chamber. 
Stieda has not made use of this, and has not at all correctly 
represented the membranes which form the wall of the atrial 
and neighbouring chambers. The brown pigmented mem- 
brane can be traced in a vertical right and left section from 
the raphe in the middle line of the ventral wall (2 of fig. 4, 
Plate I, Stieda) along the muscular floor, over the swelling 
generative sac, up to the muscular lateral wall (epipleur), 
whence it passes across to the suspended pharynx. Here it can 
be followed, clothing each bar on the outer or atrial side, and 
also continued across the short transverse bars, but plainly 
_ and certainly enough adsent (as is all tissue or membrane) 
from the space between successive bars. It may be followed 
down to the lower border of the pharynx, and so traced over 
the structures of the right hand as of the left hand side of the 
section. ‘lhe same membrane is reflected over the coecum. 
Thus this brown pigmented membrane, which may be called 
the atrial tunic, gives aready means of determining the boun- 
daries and connections of the atrial chamber. It is to be 
observed that the atrial tunic is not equally pigmented 
throughout, being especially deficient in colouring matter at 
that part where it passes from the epipleural wall of the 
atrium tothe pharynx. Of its continuity there is not, how- 
ever, a Shadow of a doubt. There are other points as to the 
