400 Dr. 0. Biirger on the 



in tlie first two groups is provided with nematocysts (we were 

 able to determine the presence of rhabdites only), in the 

 third, with the exception of the parasitic Malacohdella, it is 

 armed with stylets. The proboscis varies in structure in 

 Groups I. and 11., and even in the arrangement of the layers 

 of its wall we find important variations between a EupoUa 

 and a Cerehratulus. The proboscis of Carinella is composed 

 of a circular and a strong longitudinal muscle-layer, while 

 that of EupoUa shows the opposite arrangement of a longi- 

 tudinal and a circular layer. In Cerehratulus^ again, we find 

 that the proboscis repeats the structure of the musculature of 

 the body-wall, and we get a longitudinal, a circular, and a 

 longitudhial muscle-layer. In Carinella the nerves of the 

 proboscis adjoin the circular muscle-layer, but in the case of 

 EupoUa the longitudinal layer, and in this the nerve-tissue 

 exhibits a condition which, so far as my own experience goes, 

 is only repeated in the proboscis of the Enopla, viz. that the 

 nerve-mass is not adjacent to a circular muscle-layer, as it other- 

 wise is in all our species, be they those of Carinella^ EupoUa^ 

 Cerehratulus^ Drepanophorus^ &c., wherever we find that the 

 nerve-mass has a constant position, whether in the form of a 

 nerve or of a nerve-sheath. In the proboscis of Cerehratulus 

 the nervous plexus, derived from the expansion of the two 

 nerve-cords, adjoins the circular muscle-layer on the inner 

 side. The proboscis of the Enopla exhibits a precisely 

 similar structure, consisting of circular, longitudinal, and 

 circular layers. The nerve-cords are imbedded in the longi- 

 tudinal muscle-layer, dividing it into two sheets. The aper- 

 ture of the proboscis-sheath, however, is not, as has often been 

 assumed to be the case, terminal in position ; on the contrary, 

 it is in all forms subterminal and ventral. This is clearly 

 expressed even in Carinella, where the tip of the head pro- 

 jects beyond the aperture of the proboscis-sheath. Another 

 organ, however, the cephalic gland, does open terminally to 

 the exterior. 



A comparison has been suggested between the proboscis of 

 Nemertines and the so-called proboscis of the Turbellaria 

 Proboscidea, a terminally placed retractile and extensile 

 sense-organ. Yves Delages * and Salensky f are among the 

 more recent advocates of this theory. In opposition to this 

 we may repeat once more that the aperture by which the 

 Nemertine proboscis is extruded is by no means terminal, 



* Yves Delages, " Etudes bistologiques sur les Planaires Rhabdocceles 

 Acceles,'' Arch, de Zool. experiment, et g^n^r. s6r. 2, t. iv. 1886. 



t Salensky, "Bau u. Metamorphose d. Pilidiums," Zeitschr. fiir wiss. 

 Zoologie, Bd. xliii. 1886. 



