200 K. E. VON BAER. — PHILOSOPHICAL FRAGMENTS. 



ends of the body nor by the centre and periphery, it is almost 

 always wanting in symmetry ; the egestive pole lies almost 

 always to the right of the ingestive pole. The opposite relation 

 is so rare, that it has been called inverted. On the other hand, 

 the egestive pole is sometimes very near the ingestive one, 

 sometimes is removed far from it, so that it approximates the 

 posterior end of the body. Since the digestive track is always 

 determined by these two poles^ it is more or less curved. In its 

 simplest form the track is simply looped, as in Plumatella, 

 When the aHmentary canal elongates, it rolls up spirally in the 

 middle, and the spiral apparently follows determinate laws. The 

 commencement of the intestinal canal thus appears always to lie 

 under the portions which follow. 



The main current of the blood follows a curve which does not 

 lie in the middle line of the animal. If the spiral of the intestine 

 be wound in one plane, a certain symmetry is so produced, as in 

 the discoidal univalves and the equivalve bivalves; but this 

 symmetry is only very unimportant, and it might be called 

 almost accidental, since it is often wanting in nearly allied 

 animals. 



The nervous system is composed of scattered ganglia, which 

 are united by filaments into a network. The larger ones are 

 collected round the pharynx. The psychical apparatus is very 

 little developed, and the organs of sense are late in arising. 

 Motion is very slow and weak. From the absence of articula- 

 tion, the muscles are interwoven in all directions, and operate 

 upon every point by single bundles ; thence arise contractions 

 in all directions. Since, in accordance with the massive arche- 

 type, the secretory organs, which in other archetypes appear as 

 tubes, coil up in this, the glands are abundant and large. The 

 plastic organs in general are those which are earliest and most 

 completely developed ; and thence this archetype might also be 

 called the plastic archetype. Since it possesses neither lateral 

 nor peripheral equivalency, the body can be divided into similar 

 segments neither in one nor in many planes. Neither can any 

 straight axis be demonstrated in it round which the organization 

 is distributed ; it is rather determined by manifold curves. 



In the Vertebrata we find a fourth archetype. It is, however, 

 in a manner constructed out of the preceding archetypes. We 



