Anatojnt/ a)i<1 Histology of Kemertinea. 401 



but that the spot wliere tlie proboscis of Oonvohda Scliuhiiy 

 for example, is ]ilacecl, is occupied by the cephalic gland in 

 Nenicrtines. 



In addition to this, the relation in which the mouth and tlie 

 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 tact that we must regard the pro- 

 boscis as a species of pharyngeal ajiparatus — as a pharynx, 

 which is now no longer enclosed in tlic 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 Prostliiostomum 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 papillfe and with jaws, and is a 

 structure which, especially in the case of the Eunicidty, 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 blastococle 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, "Contribution to the Embryology of the Nemcrtea," 

 Q. J. M. S. vol. xxvi. 



Ann. cC- Mag. N. Hist. Ser. 6. Vol. vi. 29 



