OF ANIMALS. 55 



pens that one or more plates, now and then horny, but oftener 

 calcarious, is formed in its thickness. This substance is ordi- 

 narily sufficiently large to enable the animal to cover itself 

 completely with it: this is what we call a shell. Many are 

 deprived of eyes, some have rudimental ones, and those of 

 others are highly developed. Their nervous system consists 

 in medullary masses dispersed throughout the body, the chief 

 of which is situated across the oesophagus, which it surrounds 

 with a nervous circle. They have but little instinct, and for 

 the most part inhabit the water. Besides this they present se- 

 veral differences of organization, some assume that of the ra- 

 diata, others that of the articulata, and a third by the complex 

 nature of their organs approximate to the vertebrata. 



47. The acephala without shells or the tunicata have some 

 resemblance to the radiated animals. Some are collected in one 

 common body, like polypi ; among them some are disposed in a 

 star, the anuses being in the centre, and the mouths at the cir- 

 cumference; others form a cylinder in which end the anusses, 

 the mouth opening externally, while others have the viscera 

 prolonged in a common mass, and the radiated mouth and the 

 anus approximated towards the free extremity of the body. 

 A fourth kind only remains united, long after birth: these 

 when they are separated exhibit the form of a contractile 

 tube open at both ends, and in the thickness of which are 

 placed the viscera. Finally, there are others fixed to rocks, 

 which resemble two tubes, the one enclosed within the other, 

 and between whose parieties they cause the water to pass. 

 They all possess an alimentary canal with two openings, bran- 

 chias, a liver, heart and ovaries or internal buds, which, 

 without copulation, produce living young ones; they have all 

 ganglions and nervous filaments. 



48. The cirrhopoda constitute a small group of animals, 

 intermediate, between the mollusca and the articulata. Their 

 short body without a head and transverse rings, is fur- 

 nished with a tunick and multivalve shell which resemble 

 those of the acephala; the mouth has lateral jaws, and along 

 the belly there are articulated appendages with a horny skin, 

 disposed in pairs, resembling the natatory feet of the tail of 



