BRISTLE-WORMS 431 



by the contractions of the muscular body-wall aided by the setae, 

 which are comparatively few in number. The pharynx is not 

 armed with horny jaws. 



A common fresh-water form is the Red River- Worm {Tubifex 

 rivulorum), a small creature often to be seen in ponds, partially 

 imbedded in the mud. Large numbers often occur together, 

 making up conspicuous red patches, that disappear when the 

 mud is disturbed, which means that the worms have withdrawn 

 themselves into it. 



Earth-Worms are found in almost all parts of the world, and 

 there are a number of British species, of which the Common 

 Earth- Worm {Lumbricus herculeus) is usually taken as a type. 

 The head consists of a mouth-segment and a head-lobe, and is 

 entirely devoid of eyes or feelers. The trunk is divided into 

 a large number of clearly -defined segments, and not very far 

 from its front end is a band-like thickening, the girdle (clitellum), 

 which has to do with the formation of capsules in which the eggs 

 are enclosed. The popular notion is that it is the result of injury 

 by the spade. If an earth-worm be passed, tail first, between the 

 finger and thumb, a distinct feeling of roughness will be experienced, 

 due to the presence of four double rows of setae, the tips of which 

 just project above the skin. 



Order 3. — Simple Segmented Worms (Archiannelida) 



A small number of simple marine worms constitute this order, 

 and it is probable that to some extent at least they possess the 

 characters of the ancestral stock from which all segmented worms 

 are descended. It must not be forgotten, however, in this 

 connection, that simplicity of structure is often the result of de- 

 generation, and it is possible that some of these creatures may 

 be on the down-grade. There is a small head-lobe, and most 

 of them have a fairly long trunk. The segmentation of the body 

 is well marked by the presence of encircling grooves, and also by 

 rings of cilia, which serve as locomotor organs in place of setae 

 and foot-stumps, here entirely absent. Probably the most primitive 

 of the Archiannelids is the minute Dinophilus, of which more than 

 one species occur on our coasts. It presents certain points of 

 affinity to the unsegmented worms, which will be considered 

 later. Polygordius is a slender cylindrical red worm found. 



