CLASSIFICATION OF THE ANIMAL KINGDOM. 181 



lower orders slight variations are met with. In Chiton, for instance, we 

 find two auricles and a single ventricle. The organs of reproduction also 

 vary very considerably in the different genera. The lowest orders are, 

 like the Conchifera, all hermaphrodite and self-impregnating; the more 

 highly-organized Pectinibranchiata are all diaccious; others, again, as the 

 Heteropods, so called from having their foot so compressed as to be only 

 serviceable for swimming purposes, are simply hermaphrodite. Lastly, the 

 nervous system of the animals we are now considering also necessarily varies 

 with the more or less developed organic structure of the different orders. 

 Throughout the whole class it presents the characteristic scattered condition 

 of the Heterogangliate division. The principal centres are the supra-ceso- 

 jhagal mass, or brain, which gives off radiating nerves to the organs of 

 the senses, a large sub-oesophagal mass connected with the former by 

 lervous filaments, and other smaller ganglia in different parts of the body 

 which supply the viscera, and are also connected with the brain and each 

 other by communicating filameuts. In the higher orders these ganglia, 

 mwever, become condensed and concentrated, and, as in the Common Snail, 

 exhibit only two great nervous masses, the supra and sub-oesophagal 

 ganglions, with radiating filaments to the various organs of sensation and 

 the viscera, etc. 



The sixth class, the Pteropoda, consists of animals which live on the 

 surface of the waves, and swim abgut by means of two small instruments 

 placed on each side of the neck, resembling wings. They are found some- 

 times in immense quantities, and form the food of Cetacea and other 

 inhabitants of the ocean. The heads of these animals are composed of 

 various complex parts. The mouth is a simple triangular opening, armed 

 with a very singular dental apparatus, and a tongue covered on the upper 

 side with sharp recurved hooks. Round this mouth are placed six small 

 hollow tubercles, each one containing a number of little suckers, which can 

 be protruded at pleasure, and various other tentacular appendages. The 

 digestive apparatus consists of an oesophagus, a wide stomachal cavity sur- 

 rounded by the liver, and an intestine which turns upwards to the left 

 side of the neck, where the anal aperture is situated. The nature and 

 position of the respiratory apparatus has not been yet satisfactorily made 

 out, but the animals possess a heart with a single auricle and ventricle. 

 "The generative system resembles," says Professor Jones, "in all essential 

 particulars that of the most highly-organized Gasteropods, and, as in them, 

 composed of a complete set of male organs, as well as of ovigerous 

 riscera." These creatures, besides organs of touch, are possessed of a pair 

 of very complete eyes situated on the back of the neck; and, as we might 

 expect from the perfection of these organs, and the completeness of their 

 general structure, though, so insignificant in size and occupying so small 



VOL. VII. 2 B 



