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ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



unusually bulky worm which attracts attention, when cast up on the 

 shore, by the magnificent iridescence of its long chaetae, there exist quite 

 similar elytra to those of Polynoe (Fig. 71, B, d.c), only in this case they 

 are not visible in surface view, being hidden away under curious tough 

 felt. The nature of this is seen by studying a transverse section of the 

 worm like that shown in the diagram (Fig. 71, B). It is seen that the 

 notopodium does not project from the surface but its position is indicated 

 by its chaetae. Of these there are three different sets. Furthest out 

 are the long fine iridescent chaetae (i), which give the creature its 

 characteristic appearance. To the inner side of these are short stiff 

 black chaetae (2), and amongst these are produced the third set of chaetae 



Fig. ;i. 



Diagram to illustrate the parapodium of Polynoe (A) and Aphrodita (B) (from Benham, Cambridge 

 Natural History), ac, Acicula ; d.Ct dorsal cirrus ; u.c. ventral cirrus, i. Iridescent chaetae ; z, stiff 

 black chaetae ; 3, felt. 



which break up into their constituent fibres and become felted together 

 to form a roof (3) which covers over the elytra. 



In many cases the Polychaete shows peculiarities clearly related to 

 peculiarities in its mode of life. For example Tomopteris lives a pelagic 

 existence, swimming about in the surface waters of the sea. Its para- 

 podia are long and paddle-hke : their chaetae have disappeared except 

 two on each side at the head end. As in so many pelagic creatures 

 the body has developed a glassy transparency which makes it almost 

 absolutely invisible when alive in the sea-water. Many Polychaetes 

 live in burrows, such as the ordinary Lug-worm {Arenicola) which is 

 responsible for the numerous heaped-up sand castings so commonly 

 seen on flats of sandy mud between tide-marks. In it the parapodia 

 are reduced in size so as to be quite inconspicuous and the neuropodium 

 and notopodium are some distance apart. A number of the notopodia 



