ANNELIDS 199 



of worms, the Acanthocephala and the Nematomorpha. The 

 former class, as indicated by the name, include the spiny-headed 

 worms. These are cylindrical worms of peculiar anatomy, 

 notable for the complete absence of a digestive tract as in the 

 tapeworms. The head is furnished with a proboscis which is 

 armed with rows of thornlike hooklets. The adults live in the 

 digestive tracts of their hosts, burying the thorny proboscis 

 deep into the mucous membranes. They have a complex life 

 history, the larval stage being passed in insects of various kinds. 

 Several species are occasionally but rarely found in man. 



The class Nematomorpha is comprised by the " horse-hair 

 snakes," so called from the popular belief that they develop from 

 horse hairs which fall into water. They are exceedingly long 

 slender worms, usually parasitic in insects. Occasionally they 

 are accidentally swallowed by man with drinking water and are 

 usually vomited, much to the surprise and horror of the tempo- 

 rarily infected person. 



Annelids. — The most highly organized phylum of worms is 

 Annelida, including the segmented worms or annelids. In three 

 important respects these worms are the first animals in the scale 

 of evolution to develop the type of structure characteristic of 

 the vertebrate animals, consisting, namely, in a division of the 

 body into segments, in the presence of a blood system, and in 

 the presence of " nephridia " or primitive excretory organs of 

 the same fundamental type as are the kidneys of higher animals. 

 In addition the digestive system is highly developed and there is 

 a well-developed nervous system distinctly concentrated in the 

 head. In some annelids the sexes are separate, while in others 

 both reproductive systems occur in the same individual. 



At least three classes of Annelida are usually recognized, namely 

 the Archi-annelida, including a few primitive marine forms; 

 the Chsetopoda, including the worms which are furnished with 

 bristles or setae, such as the earthworms and marine sand worms; 

 and the Hirudinea or leeches, in which there are two suckers but 

 no setse. There are a number of other groups of worms which 

 many zoologists include with the annelids, but as their systematic 

 position is doubtful and as they include no parasitic forms they 

 need not be mentioned here. The only class of annelids which 

 includes parasites of man are the Hirudinea or leeches. These 

 animals superficially resemble flatworms but they can readily 



