THE BRISTLE-WORMS. Q\) 



worm may be obtained by digging in the sand where the 

 castings are abundant. Except by the intervention of a 

 strong arm and a powerful spade, however, the process is 

 not very easy, and the simplest plan is usually to invoke 

 the aid of a fisherman amateur or professional. The speci- 

 mens chosen should be not less than seven or eight inches 

 in length, and should be obtained uninjured and in the 

 living condition. The worms have an exceedingly well- 

 developed blood-system, and are full of blood, a fact which, 

 combined with the delicate body-wall, makes it not very 

 easy to obtain perfectly uninjured specimens. 



The lob-worm will not be found easy to keep alive for 

 any length of time, but it will live for a day or two if 

 placed in a vessel with wet sand, and there some of its 

 habits can be readily observed. The way in which it moves 

 is especially interesting, but before describing this we must 

 just glance at its external characters (see Fig. 10). 



In studying these we are at once struck by the marked 

 contrast with Nereis, especially in the condition of the 

 parapodia. Let us recall for a mome*nt the functions of 

 these structures in Nereis; they are locomotor, respiratory, 

 sensitive. Now the lob-worm is more or less sedentary, so 

 that we should expect that the parapodia, in so far as they 

 are locomotor organs, will show reduction. Correlated with 

 the sedentary habit we have here, as always, a greater 

 difficulty in breathing, and so we have the development of 

 special respiratory organs, the gills. Again, a sedentary 

 animal has a more limited environment than an active one, 

 and is less likely to have well-developed sense-organs, so 

 that we should expect to find that the parapodia have to a 

 large extent lost their sensitive nature. The first glance at 

 Arenicola will show that with loss of function there is also 

 degeneration of structure. Its parapodia are mere rudiments, 

 little tufts of bristles. The characteristic sensitive cirri of 

 Nereis seem to be absent, we say seem to be advisedly, for 

 the tuft-like gills in the middle region of the body are in 

 reality the metamorphosed dorsal cirri, which have lost their 

 sensitive and taken on a respiratory function. With the 

 exception of these gills, in reality a specialised portion of 

 the parapodia, the parapodia of Arenicola are very greatly 

 reduced, and do not function as locomotor organs. 



