298 Account of Professor Ehrenherg's 



distinct head. In the Rotatoria it may be defined the part of 

 the body placed between the head and anus. 



The part named the tail is an elongation of the body behind 

 the anus ; it differs from the tail of other animals in being as it 

 were a prolongation of the ventral and not of the dorsal part of 

 the body, so that the anus is placed above its base, and not be- 

 low as usual. It is sometimes truncated, at other times forked 

 at its extremity, and is, in most cases, furnished with one or 

 more suckers, by which the animals can fix themselves to sur- 

 rounding objects. 



C. External org-ans or appendages of the infusoria. 

 These Dr Ehrenberg classes under three heads, viz. 1. Simple; 

 and, 2. Compound organs of motion ; and, 3. Other appendages 

 and organs which do not serve for motion. Of the first sort, 

 there are none more remarkable than what are called variable 

 processes, which are observed in some of the Polygastric infuso- 

 ria, and which are altogether temporary and transient in their 

 existence. They are the result of a faculty which the animals 

 possess of elongating the substance of their bodies at one or 

 more points, in the form of a tube or lobe-like process, and con- 

 sequently of giving rise to many proteus-like changes of figure, 

 for which preceding observers have often been puzzled to ac- 

 count. The mode in which this phenomenon takes place may 

 be well seen in the Amoeba : the animal allows a small part of 

 the parietes of its body to become relaxed, while it contracts 

 them forcibly in the rest of their extent ; by this means the in- 

 ternal contents, or viscera, are urged against the relaxed part 

 and distend it into a bag or hollow process of variable form, the 

 cavity of which they occupy. In this manner, the whole gra- 

 nular substance within the body and the stomachs, with their con- 

 tained food, are sometimes forced into such a protrusion, which, 

 in its mode of formation, might not inaptly be compared to a 

 hernia. In the Amoeba such variable processes may be formed 

 at any point ; in other cases, as in Arcellina, they are observed 

 to occur only on the fore part of the body, and do not receive 

 any part of the alimentary canal, but seem to be protruded by 

 means of a pellucid fluid. The next kind of simple organs of 

 motion arc the setae or bristles. These appendages arc im- 



