406 Dr. 0. Burger on the 



of the body-wall we have the lateral indentations known as 

 cephalic pits in the majority of the representatives of Group II. ; 

 these supply the place of a canal in bringing the lateral organs 

 into communication with the outer world. We have yet to 

 mention the existence of a pair of lateral organs in the neigh- 

 bourhood of the nephridio-pores of Carinella. 



While the Nemertines, owing to their plexus-like epithelial 

 and subepithelial nervous layers, give grounds even for a 

 reference to the Ccelenterates (a vista opened up by Hubrecht) , 

 nevertheless the central nervous system shows so high a 

 degree of development, in the stoutness of its central sub- 

 stance, of its ganglionic coat (so widely and so sharply 

 differentiated from it), and of the twofold membranous and 

 fibrillar elements of its sheath, that it equals the Annelids in 

 this respect. The appearance of a second sheath surrounding 

 the central substance is of especial importance. An inner 

 neurilemma of this kind, which interposes itself between the 

 coat of ganglion-cells and the fibrillar substance, has been 

 identified and described by Hermann * in Hirudo also. The 

 tissue, however, which has been styled by many authors an 

 inner neurilemma, does not correspond to the inner neuri- 

 lemma of Nemertines. For the term has been applied to the 

 finely fibrillar elements of the sheath of the ganglion-cells 

 (Nansen f), or to a membranous sheath which surrounds the 

 nervous elements, ganglion-cells, and central substance of the 

 ventral cord of certain Annelids, and which, as an inner neuri- 

 lemma, has been contrasted with an outer one, which envelops 

 an inteimediate mass lying between the two membranes 

 (Leydigt, Andrea} §). 



In other respects the connexions which can be made out 

 between the brain of Nemertines and that of Annelids are 

 many in number. I may instance in particular the fact which 

 has lately been more and more insisted upon, viz. that the 

 ganglionic coat consists almost exclusively of unipolar 

 ganglion-cells, and lastly, but by no means least, the occur- 

 rence in Nemertines also of neurochord-cells and neurochords. 



Whether we are justified in placing the brain of Nemer- 

 tines absolutely on a level with that of Annelids appears to 

 me to be a question which must be postponed for the present 

 on embryological grounds. Salensky arrives at the following 



* Hermann, ' Das Centralnervensystem von Hirudo medicinalis.^ 



Miinclien, 1875. 



t Nansen, " Auatomie u. Histologic des Nervensystems der Myzo- 



stonien," Jenaische Zeitschr. 1887. 



X F. Leydig, ' Tafeln ziir vergl. Anatcniie,' Tiibingeu, 1864, i. fig. 9, 

 § J. Andrea', " Beitriige ziir Anatomie iiud Histologie des Sqmnculus 



nuclus" Zeitschr. fur Miss. Zoologie, Bd. xxxvi. 



