OP ANIMALS. 45 



kinds, more or less numerous, and always symmetrical; viz. 

 simple eyes, the cornea of which present only one facet, the 

 iris only one opening, and the optic nerve a single filament, 

 and compound eyes, or with man}* facets with as many pupils 

 and with as many filaments of the optic nerves. Sometimes 

 the eyes are pediculated or placed on articulated appendages. 

 The acephalous mollusca are deprived of eyes; most gastero- 

 poda have them, but small and rudimental, placed either on 

 the head or the posterior tentacula. The cephalopoda have 

 two large eyes covered with a transparent skin.' In the ver- 

 tebrated animals, the eyes are wanting in a very small num- 

 ber of species. 



29. The nervous system is unknown and seems not to 

 exist in the infusorii. The first rudiments of it are to be ob- 

 served in the radiated animals. The Hydra, among polypi, 

 possess microscopic globules the nature of which is uncer- 

 tain. But in the sea-star and in the Holothuria there are gan- 

 glia arranged in a circular form around the mouth, communi- 

 cating with each other by soft filaments, distributing others in 

 a radiating manner to the different parts of the body, where 

 some are conveyed to the external, and others to the internal 

 skin. In some intestinal worms we observe a nervous ring 

 around the mouth, whence arise two cords which extend the 

 whole length of the body. In the articulated animals the ner- 

 vous system presents a tolerably general character. There is 

 a little enlargement placed on the sesopbagus called brain, fur- 

 nishing nerves to the parts which are connected with the head. 

 Two cords, which encircle the aesophagus like a necklace, ex- 

 tend under the intestinal canal, and unite at intervals, forming 

 as many double ganglia or knots as there are rings in the body; 

 thence arise the nervas of the trunk and those of the extremi- 

 ties when any exist. The arrangement is nearly the same in 

 the cirrhipoda. In the mollusca there is a greater variety than 

 among the articulated animals. These means of communica- 

 tion, however, are ganglia united by cords, and conveying 

 filaments to the different external and internal parts. In the 

 acephala there is above the mouth a principal ganglion, im- 

 properly called brain, and another at the opposite extremity 



