Anatomy and Histology of Nemer tines. 401 
but that the spot where the proboscis of Convoluta Schulzii , 
for example, is placed, is occupied by the cephalic gland in 
Nemertines. 
In addition to this, the relation in which the mouth and the 
opening of the proboscis- sheath stand to one another, particu- 
larly as exemplified in Malacobdella , appears to me to be in- 
structive, and to point to the fact that we must regard the pro- 
boscis as a species of pharyngeal apparatus — as a pharynx, 
which is now no longer enclosed in the pharyngeal pouch as a 
division of the oesophagus, but possesses a cavity of its own. 
The structure of the pharynx, too, is precisely similar to that of 
the Nemertine proboscis, consisting as it does of circular and 
longitudinal muscle-layers, besides radial muscles. (In the 
case of Prosthiostomum sipunculus we have the following 
arrangement : — longitudinal and circular layers, radial 
muscles, longitudinal and circular layers.) The pharynx, too, 
possesses gland-cells, or, at any rate, the prolongations of 
such cells open through its walls. The pharynx is also 
supplied with nerves, in the form of a nerve-sheath. The 
pharyngeal apparatus of the Annelids, which is styled a 
proboscis, is furnished with papillae and with jaws, and is a 
structure which, especially in the case of the Eunicidae, where 
it lies in a chamber separated from the gullet, forcibly reminds 
us of the Nemertine proboscis, though owing to its position, 
ventral to the intestine, a direct comparison between the two 
is impossible. 
We find that the cavity of the proboscis-sheath in Nemer- 
tines increases in extent from the first group to the last. It 
has been regarded as equivalent to a body-cavity, and as such 
its development from the blastocoele proves it to be a remnant 
of the primitive segmentation-cavity. Hubrecht* accordingly 
terms this space an archicoele. The cavity of the proboscis- 
sheath contains free nucleated bodies, resembling blood- 
corpuscles ; it possesses an endothelium-like lining, as is the 
case with the blood-vessels, in connexion with which it is 
supposed to have arisen. 
The cavity of the proboscis-sheath may be still further 
increased by sac-like metamerically arranged evaginations. 
The Turbellaria naturally afford us no points of comparison 
with reference to the cavity of the proboscis-sheath. 
But what about the Annelids ? I venture to put forward 
the following hypothesis : — While in Annelids all the organs 
lie in a body-cavity, in Nemertines such a cavity has only 
been developed to a limited extent, embracing the proboscis 
* Hubrecht, u Contribution to the Embryology of the Nemertea/’ 
Q. J. M. S. vol. xxvi. 
Ann . & Mag . N. Hist . Ser. 6. Vol. vi. 
29 
