The Worms and some of their Posterity 



tendency to reduplication of parts in a regular series may be 

 observed in the vertebrates, as we shall see. Slight indications 

 of it are also to be found in the Nemertines. Numerous theories 

 have been proposed which derive the vertebrates from some of 

 the Articulata from the ringed worms or the Crustaceans, and 

 even from the air-breathing members ; and at first sight such 

 theories seem attractive, for in some of their more obvious 



FIG. 52. 

 I, Marine swimming ringed worm ; 2, giant centipede; 3, Peripatus. 



Photo: Martin Duncan, Ben idge, and Bastin. 



characters there is a certain resemblance between the two groups. 

 But there are also many and fundamental differences, and few 

 zoologists have accepted any hypothesis of this type. We may 

 briefly allude to some of these differences. 



In the Arthropods, where the body consists of hard and soft 

 parts, the ' skeleton ' is an external one, and encloses the soft 

 parts. Respiration occurs by means of the skin or of gills, or, 

 in air-breathing forms, by ' trachea,' which are small branching 

 tubes opening on the sides of the body. But in no case has the 

 mouth or the digestive tract any connection with the respiratory 

 system, a condition of affairs very different from that obtaining 

 in the vertebrates. The nervous system consists of a brain, 

 situated above the gullet, a nerve ring round the latter, and a 

 double nerve cord running along the body, below the digestive 

 canal. This is obviously the opposite position to that occupied 



