430 



CHARACTERS OF INVERTEBRATE ANIMALS 



region may either be much reduced and devoid of appendages, 

 or else, in the forms inhabiting firm permanent tubes, provided 

 with numerous tactile filaments and large branching gills. The 

 foot-stumps are not so well developed as in the free-living forms, 

 and the pharynx is not armed with horny jaws or teeth. 



The Lugworm (Arenicola piscatorum) (fig. 266) is well known 

 as a burrowing form highly esteemed as bait. It makes U-shaped 

 passages in the mud or sand, near which may be seen 

 coiled " worm-castings " made up of sand and undi- 

 gested food, which have been voided from the body. 

 The most striking external feature consists in the 

 presence of branching gills projecting from the middle 

 region of the body. Cirratulus is a long cylindrical 

 worm often to be found buried in the sand under- 

 neath stones. Its locomotor organs are much reduced, 

 and the dorsal cirri are slender elongated filaments 

 which project above the surface of the sand and act 

 as gills, the active wriggling movements which they 

 constantly execute giving them a resemblance to small 

 red worms, for which they are often mistaken. Another 

 common shore form is Sabellaria, which glues sand- 

 grains together into a tube. Large numbers of these 

 animals live associated together, and their tubes often 

 form compact masses of considerable extent. Every- 

 one must have noticed at the sea-side small convoluted 

 limy tubes encrusting oyster-shells or stones. These 

 belong to Serpuia, a particularly beautiful form, in 

 which the head bears two brightly-coloured bunches of 

 gill-filamentSc One of these is converted into a conical 

 horny stopper, which closes the opening of the tube 

 when the worm has withdrawn itself. Spirorbis is a 

 related but much smaller form, in which the tube is 

 rig. 266. coiled into a flat spiral. Large numbers of these 

 may often be found adhering to brown sea-weeds. 



Order 2. FEW-BRISTLED WORMS (Oligochseta) 



These are segmented worms which for the most part live in 

 fresh water or burrow in the earth. They lack feelers and foot- 

 stumps, while gills are but rarely present. Locomotion is effected 



