THE BRISTLE-WORMS. 95 



shore is generally Pulynoe imbricata, the common scale- 

 worm. If you drop your specimen into a collecting jar you 

 will notice that it wriggles its way downwards through the 

 water, after a fashion which can hardly be justly described 

 as swimming. If you go on with your collecting and re- 

 examine the worm after an interval, you will probably find 

 that the jar contains, in addition to the worm, a number of 

 small flat plates of greyish tint. These are the elytra, or 

 scales, of the worm, and it not infrequently happens that 

 the little creature will throw off every one of these within 

 a very short period of its capture. When these are gone 

 the segmentation of the body is very clearly visible, and 

 the animal looks so different that it may not be recognised 

 as the same creature. Not a few of the shore animals have 

 this power of throwing off parts of their body, apparently 

 on very slight provocation, and in cases like the present the 

 use of the habit is not very obvious. 



The Polynoids do not make satisfactory inhabitants of an 

 aquarium, nor do they generally make good preparations, 

 for it is very difficult to get a complete specimen. Never- 

 theless, a few specimens should be taken, for the animal 

 well repays examination. It is a very abundant and widely 

 distributed species, occurring on both sides of the Atlantic 

 and in Japan. It must, therefore, be very well adapted to 

 the conditions of shore life, though it is not easy to point 

 out the nature of the adaptations. 



The following general points should be made out. The 

 body is short, flattened, and has nearly parallel sides. 

 The head has three tentacles and a pair of palps, while 

 the next segment, the peristomium, bears a pair of elongated 

 cirri at each side. The remaining segments are furnished 

 with parapodia of typical form, with dorsal and ventral 

 branches bearing bristles. Now it will be recollected that 

 in Nereis the parapodia bear dorsal and ventral tactile 

 processes or cirri. What about the cirri of Polynoe? A 

 little careful examination will show you that the ventral 

 cirri are present on all segments, though except in the case 

 of the anterior segments they are short. The dorsal cirri 

 occur in the typical condition on every second segment in 

 the anterior segments, and in the posterior region are missing 

 only from every third segment. When they are absent 



