224 K. E. VON BAER. PHILOSOPHICAL FRAGMENTS. 



ends, because the whole animal part of the body is singly, and 

 not doubly symmetrical. 



Whether the anterior pair of ganglia of the Articulata is to be 

 called a brain or not, depends wholly upon the signification given 

 to the word ( brain/ It is assuredly not the organ which we call 

 brain among the Vertebrata, for this is the anterior extremity of 

 that nervous tube which is absent in the Invertebrata. It is 

 rather the most anterior pair in the series of ganglia, and since 

 these are to be compared with the spinal ganglia of the Verte- 

 brata, the so-called brain appears to be for the longitudinal type 

 what the Gasserian ganglion is for the Vertebrata. This also 

 receives nerves of sense. A great importance appears to be 

 attached to the circumstance that it lies above the oesophagus. 

 This, however, appears to me to be an erroneous view. Properly 

 speaking, it only lies in front of the oesophagus. 



If, in fact, we form a purely ideal notion of the longitudinal 

 type, the oral aperture is not placed at the anterior extremity, 

 but is directed downward, just as the oral aperture of the Ver- 

 tebrata is not situated at the anterior extremity of the vertebrate 

 type, but is placed somewhat posteriorly towards the abdominal 

 surface, for which reason a portion of the abdominal visceral 

 plates, the walls of the nose, lie in front of and above the oral 

 aperture. In the Chick it is very clear that the mouth opens 

 below. That in the Articulata the oral aperture belongs to the 

 lower half of the simple ring, is shown very clearly by the 

 Crustacea, and even by forms which exhibit the type in a less 

 altered form, as the Annelida. In the Earthworm, for example, 

 this relation of the so-called proboscis, which extends beyond 

 the oral aperture, is obvious. It contains the most anterior im- 

 perfectly developed rings. Now if in the Articulata the oral 

 aperture is in fact anterior, but yet upon the lower surface, and 

 corresponds with the most anterior extremity of the abdominal 

 surface, a pair of nervous ganglia must necessarily lie in front of 

 the oral aperture, and that they lie nearer the upper wall than 

 the posterior ganglia arises partly from the passage of the mouth, 

 partly from the very fact of its occupying the anterior extremity. 

 Very frequently it lies actually in the same plane with the others, 

 as in the Crustacea, where the mouth lies further back ; and in 

 Insects, where the head, with the oral aperture, is directed more 



