THE BRISTLE-WORMS. 91 



had a superficial justification in the fact that sedentary 

 forms have certain external characters in common. In 

 Arenicola we see, as it were, the first effects of the passive 

 life upon the organism. As the sedentary habit becomes 

 more firmly fixed, the bristles become more degenerate, 

 except when specialised anteriorly to aid in tube-building. 

 At the same time the tube becomes more and more highly 

 developed. It may consist entirely of secretion poured out 

 by the animal, or may be composed of foreign particles glued 

 together by the secretion. This secretion is present to a 

 slight extent in Arenicola^ where, as we have seen, it gives 

 a certain amount of firmness to the walls of the burrow. 

 In most tube-builders there are on the ventral surface 

 swollen areas, known as "gland-shields," which seem to be 

 of much importance in tube-formation. Though these as 

 such are not distinct in Arenicola, yet the ventral surfaces 

 of the segment lines in the middle region of the body are 

 in life much swollen, and are probably of much importance 

 in the production of the secretion used in lining the burrow. 



In looking for these glandular regions it will be noticed 

 that in Arenicola the body is closely ringed, the rings being 

 more numerous than the bristles which mark the segments. 



Having observed these points, the next step is to dissect 

 the internal organs. Pin the animal down on wood or 

 paraffin under water, with the dorsal surface that bearing 

 the gills uppermost. An ordinary pie-dish, in which a piece 

 of weighted cork or wood has been placed, makes an excellent 

 dissecting-dish, or a couple of candles may be melted in the 

 pie-dish, and the animal pinned down on the solidified 

 surface. Put the anterior pin in carefully, so as not to 

 injure any of the internal structures. Then take a pair of 

 fine scissors, and slit up the dorsal surface between the gills 

 from the head to a little behind the last gill. Pin out the 

 body-walls, and the dissection should present the appearance 

 shown in the figure. 



The first point to be noticed is the large size of the body- 

 cavity, and the absence of transverse partitions, or septa. 

 The large body-cavity is characteristic of bristle-worms in 

 general, but in most of them it is divided into numerous 

 compartments by divisions which correspond to the segments. 

 The absence of these septa is no doubt an adaptation to the 



