148 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



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



y the magnificent iridescence of its long chaetae, there exist quite 



i-lvtra to those of Polynoe (Fig. 71, B,, d.c), only in this case they 



;i>le in surface view, being hidden away under curious tough 



i'he nature of this is seen by studying a transverse section of the 



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 



Of these there are three different sets. Furthest out 



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



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



iiaetae (2). and amongst these are produced the third set of chaetae 



d.c t 



FIG. 71. 



Utatrate tin- parapodium of Polynoe (A) and Aphrodita (B) (from Benham, Cambridge 

 History), ac, Acicula ; d.c, dorsal cirrus ; v.c, ventral cirrus, i, Iridescent chaetae ; 2, stiff 



. i, f'lt. 



whu-h break up into their constituent fibres and become felted together 



nn a roof (3) which covers over the elytra. 



In many cases the Polychaete shows peculiarities clearly related to 



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



wimmintf about in the surface waters of the sea. Its para- 



"< I.n- and paddle-like: their chaetae have disappeared except 



h side at the head end. As in so many pelagic creatures 



levdoped a glassy transparency which makes it almost 



nvisiUe when alive in the sea-water. Many Polychaetes 



ich as the ordinary Lug-worm (Arenicola) which is 



responsible fur the numerous heaped-up sand castings so commonly 



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



as to be quite inconspicuous and the neuropodium 



m.I notupu.hum are some distance apart. A number of the notopodia 



