THE GENERA OF NEMERÏEANS. 203 



by fibrous commissures. Vestiges of a simple transverse 

 groove on a level with the cerebral ganglion are present 

 in Carinella as well , although , as mentioned above , no 

 ciliated canal leading to the nerve-ganglion could be detec- 

 ted in that genus. In Cephalothrix the groove too seems 

 to be absent. In the four other genera of unarmed Nemer- 

 teans : Linens Sowerby , Borlasia Oken , Cerehratulns Renier 

 and Langia mihi , the simple external opening which was 

 present in Vahncinia is represented by deep lateral fissures 

 on both sides of the head , situated in the plane of the 

 body-axis. The simple opening has here widened out into 

 these fissures which can be opened and shut at will by 

 the arrangement of the muscles of the head and which 

 are covered internally by a thick coating of very long 

 vibratile cilia. All the four genera are further concordant 

 in the deep red tinge of their nervous system, which is 

 due to haemoglobine contained in this tissue. On the 

 whole they form a very natural suborder. In the genus 

 Cerebratulus I propose to include all the species described 

 under the generic name Micrura by Ehrenberg and after- 

 wards by Mac Intosh. I could not detect anatomical dif- 

 ferences and the sole exterior difi'erence: the very small 

 caudal appendage, seems to appear under certain circum- 

 stances in true Cerebratuli as well and may hardly rank 

 as a generic character. Urticating elements , which have 

 been found in the proboscis of Carinella and Cephalothrix 

 are absent in Polia, Valencinia and all the Hoplonemertini ; 

 they reappear in numerous species of Schizonemertini 

 where they may even attain considerable dimensions. 

 The six genera of armed Nemerteans form a subdivision 

 not less natural; five of them seldom attain to a length 

 which can be called considerable in relation to their width : 

 the genus Nemertes^ however, contains worms which are 

 often extremely long and threadlike. These two subdivi- 

 sions may rank as so many sub-orders , whereas the more 

 primitive forms of Nemerteans , showing characters out of 

 which those of both these subdivisions might have gradually 



Notes trom tlie Leyden ^M^useum. 



