356 ESSAY ON CLASSIFICATION. 



type an absence of symmetry. Generally the discharging 

 pole is to the right of the receptive one, The discharging 

 pole, however, is either near the receptive one, or removed 

 from it, and approximated to the posterior extremity of 

 the body. As the tract of the digestive apparatus is 

 always determined by these two poles, it is more or less 

 arched; in its simplest form it is only a single arch, as in 

 PlumateUa, When that canal is long, it is curled up in 

 a spiral in the centre, and the spiral probably has its defi- 

 nite laws. For instance, the anterior part of the alimen- 

 tary canal appears to be always placed under the poste- 

 rior. The principal currents of blood are also in arches, 

 which do not coincide with the medial hue of the body. 

 The nervous system consists of diffused ganglia, united 

 by threads, the larger ones being around the oesophagus. 

 The nervous system and the organs of sense appear late; 

 the motions are slow and powerless. 



IV. The Vertebrate Type. This is, as it were, composed 

 of the preceding types, as we distinguish an animal and a 

 vegetative system of the body, which, though influencing 

 one another in their development, have singly a peculiar 

 typical organization. In the animal system the articula- 

 tion reminds us of the second type, and the discharging 

 and receiving organs are also placed at opposite ends. 

 There is, however, a marked difference between the Arti- 

 culates and the Vertebrates, for the animal system of the 

 Vertebrates is not only doubled along the two sides, but 

 at the same time upwards and downwards, in such a way 

 that the two lateral walls which unite below circumscribe 

 the vegetative system, while the two tending upward sur- 

 round a central organ of the animal life, the brain and 

 spinal marrow, which are wanting in Invertebrates. The 

 solid frame represents this type most completely, as from 



