32 GUSTAF EISEN, ON THE ARCTIC OLIGOCH.ETA. 



believe, was the first to perceive its importance as a character, and the principal cha- 

 racter of his Enchytrceus galba is based on the convexity of its supra-oesophagial ganglion. 



Since that time both Ratzel and rayself have described species with a similarly 

 shaped cerebral ganglion, but always thought that it furnishes only secondary or species 

 characters. With our enlarged knowledge of species our definitions of thera must natur- 

 ally be narrower and sometimes minute, and at least with regard to the genus Enchy- 

 trceus no external characters are of any value, neither in distinguishing genera nor 

 species. 



I believe that Leydig considers the convexity of the supra-cesophagial ganglion 

 as an indication of a higher development or as a further differentiation of the va- 

 rious parts of which that ganglion was originally (speaking of its phylogenesis) com- 

 posed. Being of the same opinion I naturally conclude, that the emargination of the 

 said ganglion indicates a differentiation in a less degree, a coalescence or a centrali- 

 sation not so far advanced as in more convex forms; and accordingly I hold that those 

 species are the least developed which have the deepest emargination of the ganglion, 

 and on the contrary that those with a shallower emargination are higher developed in 

 proportion, and finally that those with a convex margin are the highest of all. And 

 experience teaches us also that generally the whole organisation corresponds with the 

 development of the principal organ, at least concerning each genus considered by it- 

 self. For instance, those species of the genus Archienchytrceus which have the deepest 

 emargination of the said ganglion, have also other principal organs developed in a less 

 degree than species with a less emarginated ganglion. As the lowest on the scale of 

 development I should consider Archienchytrceus Levinseni, and, on the contrary, Neo- 

 enchytrceus Ratzeli may be taken as the highest developed of all. And between these 

 two extremities there are all grades of development. 



The cerebral ganglion of Mesenchytrceus has a form intermediate between those of 

 Archienchytrceus and Neo enchytrceus, its margin being nearly straight. As, however, some 

 others of its principal organs differ greatly from those of the said genera, I do not 

 think that the former genus has originated from Archienchytrceus, or is strictly an 

 intermediate form, but rather a side-branch of the same »phylogenetic tree». 



Also within every genus there is a great difference as to the development of the 

 cerebral ganglion. In some of the species of Archienchytrceus the cerebral ganglion is 

 cordate, and the emargination is very deep, as I find it in A. Levinseni and tenellus. In 

 A. lamjoas and Dicksoni the same ganglion is oblong and the emargination shallower. 

 The four species A. gemmatus, nasutus, affinis and ochraceus form a group by them- 

 selves, their ganglion being nearly quadrangular and the emargination about the same 

 in all species. A. nasutus has a ganglion broader in front, the two ends of the other 

 three species are of about equal width. In A- nervosus the ganglion is very large and 

 cordate and its emargination is also very deep. But in the species nearest allied to it, 

 A. profugus, we find a ganglion more res emD ];ng that of A. gemmatus. 



In Mesenchytrceus there are two different forms of the cerebral ganglion, one consi- 

 derably broader than long (M. primcevus), one on the contrary longer than broad {M. 

 mirabilis and falciformis), but all resembling cach other in having a deeply ernargi- 



