54 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



some information of what is going on in the world 

 around it. 



General sensibility of this kind is, however, soon 

 found to be insufficient for the needs of the organism ; 

 sight and hearing are possessed, no doubt, by lower 

 forms, but we shall soon find creatures with elaborate 

 eyes, and well-defined auditory organs, while obscure 

 indications of an olfactory sense are, a little later, 

 to be detected. 



The organisation of the ringed worms or Amui- 

 lata attains its highest degree of complexity in the 

 free-swimming marine forms. Here the ringed body 

 has on most of its metameres a single or double pro- 

 jection on either side (parapodium), from which 

 there project a number of bristles (set) ; at the 

 anterior end, the tentacles are aided by a number of 

 elongated feelers, and a pair of well-developed eyes, 

 and sometimes, too, auditory vesicles are to be found 

 there. The mouth is provided with strong horny 

 denticulated jaws, which are moved by special 

 muscles, and which serve to break up the food; dif- 

 ferent parts of the digestive tract take on different 

 functions, and pouches, which may again be branched, 

 sometimes appear at the sides. A fluid circulates 

 through the body in a system of closed vessels, and 

 some of these vessels have their walls provided with 

 muscles by means of which the current, which is 

 always regular in direction, is propelled onwards. 

 At the sides of the body thin outgrowths of its wall 

 serve as gills (branchiae), and most of the meta- 

 meres are provided with a pair of coiled tubes which 

 open into the spacious crelom, and also to the 

 exterior ; these are the renal organs (nephridia). 



The division of ringed* worms in which the setze 

 are numerous on each parapodium is called the 

 Polychseta; of these, some, like the sea-mouse 

 (Aphrodite), Polynoe, and Nereis are free-swimming, 



