THE BRISTLE-WORMS. 93 



the introvert is retracted, but carries with it a certain amount 

 of sand, which, be it remembered, contains the animal's food. 

 The process of burrowing is thus aided by the removal of part 

 of the sand, while the power of distending the body, espe- 

 cially in the anterior region, facilitates the progress of the 

 animal. This power is associated with the absence of septa, 

 and we thus see how deeply habit affects structure, and 

 therefore how it is that the sedentary forms show such an 

 apparently close relation to one another. It is one of the 

 most difficult tasks of philosophical zoology to distinguish 

 between resemblances in animals which are due merely to 

 adaptation to a similar mode of life and those which are due 

 to common descent. 



As to the other structural peculiarities of Arenicola, 

 notice the large glands (gl) opening into the intestine, the 

 abundant blood-supply to the gills, the ventral nerve-cord 

 (n), seen by pushing aside the alimentary canal, and the six 

 pairs of kidney tubules, or nephridia (ne), in the anterior 

 segments, which open from the body-cavity to the exterior. 



Into the minute points of structure we cannot enter here, 

 but may briefly summarise the salient features of the in- 

 ternal anatomy of a Polychsete worm. All have "a large 

 body-cavity, or space between alimentary canal and body- 

 wall, and this is usually divided into chambers by cross 

 partitions. The alimentary canal runs straight down the 

 body, and has anterior and posterior openings (contrast sea- 

 anemones and their allies). There is a ventral nerve-cord, 

 and typically a pair of kidney tubes to each segment, but 

 these are often reduced in number. 



In classifying worms the most important points to be 

 noticed are the shape of the head and the nature of the 

 feet, or parapodia, and the bristles. Our British Polychsetes 

 are very numerous, so that we can select only a certain 

 number. Those selected are those which are fairly common 

 at most parts of our coasts, and are of sufficient size to be 

 examined and identified with a lens or simple microscope. 

 The more minute forms, though often of great interest, are 

 beyond our scope. Even with this limitation, however, 

 the worms form a difficult group, and their recognition can 

 never be made easy ; but their diversity of habitat renders 

 them a group of extraordinary interest. Many of them are 



