CHAPTEE XI 



MARINE WORMS 



SOME groups of animals are so well defined that the individual 

 species contained in them can be assigned their proper place 

 without any difficulty, the main characteristics by which the group 

 is distinguished running with more or less precision throughout the 

 series ; but, unfortunately this is not the case with the ' worms,' 

 which constitute the sub-kingdom Vermes. Here we have a most 

 heterogeneous assemblage of animals, collectively exhibiting 

 exceedingly wide variations in both form and structure. 



We have already referred to the sea cucumber as wormlike in 

 form, and this creature is only one of a large number of wormlike 

 animals that are not worms ; and it is also a fact that a considerable 

 number of the worms are not wormlike. It appears as if the 

 sub-kingdom Vermes were a kind of receptacle into which we may 

 throw almost any invertebrate animal that does not readily fall in 

 line with the general characteristics of the other important groups ; 

 for in it we have such a varied assemblage of creatures that, 

 speaking of them collectively as worms, it becomes most difficult, 

 if not absolutely impossible, to say exactly what a worm is ; and 

 it is a question whether the sub-kingdom ought not to be divided 

 into at least two or three groups of the same standing. 



This being the case we can hardly give a satisfactory summary 

 of the characteristics of the group, and therefore it must be under- 

 stood that in our attempt to do so we unavoidably exclude some 

 forms that belong to it according to our present system of classifica- 

 tion. This being remembered, we will define worms as soft- 

 bodied and elongated animals, exhibiting a bilateral symmetry 

 (that is, having appendages and organs arranged symmetrically on 

 each side of a plane extending from the dorsal to the ventral 

 surface through the centre of the body), and with the body usually 

 divided into a succession of segments, each of which resembles the 



