ALLIES OF THE EARTHWORM 235 



worm-like form. Many animals that have such a form have 

 been taken out of that branch of the system and placed in 

 another group, because their discovered relationship required 

 it. On the other hand, animals like the brachiopods and the 

 polyzoans were once classed with clams, mussels, and snails, 

 because all possessed a shell ; they are now thought to be 

 more nearly related to animals like the earthworm and other 

 annelids than they are to the clams. 



So true is it that no single characteristic of structure can 

 be found that tends to unite the organisms classed as worms 

 into a well-defined group, that systematists are accustomed 

 to say that Vermes include all animals that are not clearly 

 members of some other large group. This does not indicate 

 any imperfection in the bodily structure of the members of 

 the group ; it means that our knowledge of animal morphol- 

 ogy is broken and incomplete, partly from lack of complete 

 investigation, but chiefly because some organisms that must 

 have filled up the "gaps" in the chain of relationships have 

 disappeared entirely, leaving no trace even in fossils. The 

 little basis we have for grouping all the forms discussed in 

 Chapters XVI and XVII into the group Vermes is found in 

 the tendency of the body to show a greater or a less degree 

 of metamerism (division into somites), either in the adult or 

 in the embryo. In these somites the characteristic excretory 

 organ, the nephridium, most highly developed in Nereis and 

 Lumbricus, is nearly always present. In some of the mem- 

 bers of the group several pairs of nephridia occur with no 

 other evidence of metamerism. Whenever appendages occur 

 among Vermes they are never jointed, as they are in Arthrop- 

 oda. Perhaps the strongest indication of relationship exist- 

 ing between widely differing members of the groups, as, for 

 example, Annulata and Molluscoida, is to be found in the 

 great similarity of a certain stage of the free-swimming larvse 

 of the two phyla. 



