260 Analytical Notices of Books. 



little has since been added to the labours of that accurate and indefatiga- 

 ble observer. He asserts, however, that the Danish naturalist, skilled as he 

 jvas in minute observation and in designing and describing the objects of 

 his study, had not the smallest idea of the true principle of classification; 

 and that conseqnently, in the present as in many other instances, he 

 brought together animals widely distinct from each other in every essential 

 character, and belonging to very different degrees of organisation. Dr. 

 von Baer contends that the genus Planaria should be restricted to those 

 species which are flattened and furnished with two openings beneath on 

 the posterior half of the body; and arranges this group along with the 

 TrematodcB of Rudolphi, to which he considers them properly to belong. 

 As for the remaining species enumerated by Muller, he maintains that 

 they form several distinct genera, occupying different and distant places 

 in the natural system. He describes at length, so far at least as the ex- 

 tremely delicate texture of their bodies would admit of examination, the 

 organisation of four species of true Planarice; and notices a species of 

 Cyclidium, and another animal, probably the Vorticella hamata, Miill., 

 as parasitic on the animals of this genus. 



In his concluding memoir Dr. von Baer enters more fully into the 

 question of classification, to which he had only cursorily referred in 

 his previous articles. He contends that the classes of Infusoria and 

 Entozoa require to be altogether banished from the natural arrangement 

 of animals, inasmuch as they are established on no common, exclusive, 

 or intelligible characters, and consequently admit of no real or scien- 

 tific discrimination. He further maintains that the leading types of 

 animal organization have their origin in its lowest grades, from which 

 each of them proceeds in a distinct and uninterrupted series, through 

 the gradual developement of its organs, to its highest or central point 

 of existence, giving off by the way numerous branches which frequently 

 anastomose with similarly aberrant forms of the neighbouring types. 

 Thus for instance he considers Lineola, the name by which he designates 

 the simplest form of Vibrio, the mere animated filament, as the repre- 

 sentative of Annulosa among the Protozoa. From this he ascends 

 through Vibrio, which he characterizes as a living tube producing ova, 

 to Filaria, Gordius, and Nais, in which there is a gradual develope- 

 ment of skin, vessels, and nerves. The simple form begins in these to 



