296 POLYCHAETA 



CHAP. 



part in the display, and no doubt the phosphorescence lias, like 

 the colour, a " warning " purpose. 



The production of the light in these various forms is apparently 

 due to two different processes. In some cases, e.g. Chaetopterus, 

 Syllids, Terebellids, it appears to be due to the oxidation of 

 certain cell contents which are discharged more or less freely on 

 irritation of the nerves ; whilst in Polynoids the phenomenon 

 is due to some purely nervous process, for the elytra have no 

 glands, but are provided with ganglia and a nervous network. 



In other worms, however, there are definite light-producing 

 organs. In Tomopteris there is on each parapodium, above and 

 below, a brightly-coloured spherical organ, which for a long time 

 was regarded as an eye, but from its structure appears to be 

 a "photogen" (Fig. 167, p. 315). The same is very likely the 

 true explanation of the segmental " eyes " of Polyophthalmus, for 

 their structure recalls that of the light-organs of deep-sea fishes. 



As many of the phosphorescent Polynoids are commensals, 

 while Chaetopterus inhabits tubes, and close allies of other 

 phosphorescent worms have no power of emitting light, it is 

 impossible to apply the same explanation of its purpose to 

 all cases alike ; in some it may be " accidental," though in others 

 it may be of definite use in warning enemies or in attracting 

 prey. 



The Food of Worms. The Nereidiformia are mostly carni- 

 vorous, and feed on small Crustacea, Mollusca, sponges, and 

 other animals; and Polynoids are even said to eat one another. 

 Many worms do not disdain various seaweeds, whilst the Spioni- 

 formia and Scoleciformia, which burrow in mud and sand, and are 

 without biting organs, swallow the mud and digest what animal 

 or vegetable debris it may contain. The Terebellids and Crypto- 

 cephala depend on minute organisms which may be driven into 

 the mouth by the action of the cilia of the gills or tentacles. 



In the case of deep-sea forms, it is an interesting fact that 

 the intestines are not unfrequently crammed with Eadiolaria 

 and Foraminifera in a fairly fresh, uninjured condition, indicating 

 that these Ehizopods do not merely sink to the bottom, but must 

 actually live there. 1 



The economic purposes to which Polychaetes are put are 

 few ; they are used either as bait for fishes or as food for man. 



1 M'Intosh, H.M.S. "Challenger" Reports, " Polychaeta, " vol. xii. p. ix. 



