Anatomy and Histology of Nemer tines. 401 



but that the spot where the proboscis of Gonvoluta Sckulzii, 

 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 sipuncidus 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 witli nerves, in the form of a nerve-sheath. The 

 pharyngeal apparatus of the Annelids, which is styled a 

 proboscis, is furnished with papillaj and with jaws, and is a 

 structure which, especially in the case of the Eunicidie, where 

 it lies in a chamber separated from tiie 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, " Contribution to the Embr^'ology of the Nemertea,'' 

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



Ann. d' Mag. N. Hist. Ser. 6. Vol vi. 29 



