Nova Acta Academics NaturcB Curiosorum. 259 



first described, one or two species hitherto referred to the latter type ; 

 and of another Filar ia. In addition to these the whole body of the 

 inhabitants of fresh-water univalves appears absolutely to swarm with 

 Cercari(E, in the sense in which that genus is now re-established and 

 restricted by Nitzsch; a fact the more remarkable as these supposed 

 Infusoria had never before been detected in the interior of the living 

 animal, from which they now appear to derive their origin. Numerous 

 forms of these singular creatures are figured and described with the 

 greatest minuteness; and Dr. von Baer enters into a long and profound 

 investigation of all the appreciable circumstances connected with their 

 origin, organisation, and affinities, as well of those still more difficult 

 and abstract considerations which bear upon the general question of the 

 apparently spontaneous production of animal life in its very lowest 

 degree of developement. 



The fourth memoir relates to a parasite found upon the gill-covers of 

 the sturgeon, and referred by Oken (who knew it only by a miserable 

 figure and description published by Alnlgaard in the Transactions of the 

 Copenhagen Society, under ihe name of Hirudo SturionisJ to his genus 

 Phylline. Dr. von Baer compares this animal with the other species of 

 which Oken composed his genus, and with those which constitute the 

 genus Tristoma, Cuv. ; and concludes thai Phylline is susceptible of being 

 divided into two subgenera, for the one of which, comprehend'ng the 

 species noticed by Cuvier, he retains the name of Tristoma ; while he 

 dedicates the other, containing the present species, to which he assigns 

 the name of Xitzschia elegans, and in all probability also the Hirudo 

 Hippoglossi of authours, to Professor Nitzsch of Halle, one of the most 

 indefatigable and successful observers of these paradoxical animals. 



A description of Poly stoma integerrimum, Rud., occupies the suc- 

 ceeding portion ; which concludes with a comparative examination of 

 M. Otto's genus Cijclocotijle. These two genera Dr. von Baer considers 

 as very closely related to each other. 



The PlanaricB form the subject of the sixth essay; and their zoologi- 

 cal history, physical characters, and natural habits are discussed at con- 

 siderable length and with much critical acumen. In' treating of the first 

 branch of his subject our authour shews how entirely ignorant naturalists 

 were with regard to these animals until the time of Miiller, and how 



