KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 15. N:0 7. 41 



ever imperfect 'and vague they may be. Of course no direct evidence of any develop- 

 ment through evolution exists, and it is only by comparing the organs of the different 

 species that we can get some hints about the relationship of the species and about the 

 primasval state of their organs. 



From reasons given above I have founcled the definition of the species on anatomical 

 characteristics only, and therefore it is with the internal organs we mostly have to 

 occupy ourselves. With the view to study the development of the species I have 

 more minutely described some of the principal organs and will now sum up the con- 

 clusions that may be derived from them. 



To begin with the brain or the supra-oesophagial ganglion, we have seen it to 

 be in very different stages of development in the different species, but its forms can 

 all be arranged in a continual series from the lowest to the highest species known. 

 The emargination of its margins is considered to indicate a lower, or a more primre- 

 val stage of development, than a convexity of the same parts. 



The most emarginated ganglion must therefore be considered the least developed 

 one (M. primcevus and A. Levinseni and tenellus), and the most convex or the highest 

 differentiated one (N. callosus and Ratzeli) must be counted the most highly deve- 

 loped. Between those extreme forms we find all the different grades of emargination 

 and convexity, forming a series as perfect as could possibly be expected considering 

 the few species hitherto known. 



Even the ventral nerve-chord shows indications of a phylogenetic development 

 corresponding to that of the supra-oesophagial ganglion, as in some species its gang- 

 lionic swellings are more developed than in others, and even in one of the highest 

 species {N. Stuxbergi) one of the ganglia in the third segment emits a side nerve, 

 something alltogether exceptional in this family. 



The receptacle is also a very variable organ, but being of more essential value 

 for the preservation of the species, it does not furnish any »good» genus characters 

 and, as could be expected, we find it in different stages of development within the 

 same genus. In each genus we, therefore, find a higher and a lower form of the same. 

 The lower form consists only of a single tube, the higher form, on the contrary, is 

 furnished with follicles, bladders or other enlargements for the reeeption of the sper- 

 matozoa. But even here interraediate forms exist between the two extremes, showing 

 an unbroken phylogenetic series. 



The base of the receptacle is nearly always surrounded by glands, which are 

 more nuraerous in species inhabiting wet localities. In species living in seawater or 

 under seaweeds these glands are very numerous, entirely surrounding the lower tube- 

 like part of the receptacle. Such species are A. profugus and nervosus, N. Stuxbergi 

 and Vejdovshyi, all species with a similar habitat. 



In N. callosus, a species with very härd epidermis, the glands are perfectly 

 wanting. 



The efferent duct is less variable, but perhaps of a higher value as a genus 

 character. In the genus Mesenchytrceus it is of a peculiar form, the tube proper 



K. Sv. Vet. Akad. Handl. Bd. 15. N:o 7. " 



