2/2 



POLYCHAETA 



worms. Amongst the Nereidiformia, the Syllidae and Hesionidae 

 possess oesophageal diverticula (Fig. 141, A, d\ which are used, 

 not for secreting a digestive fluid, but as reservoirs for water and 

 air swallowed by the worms ; and are provided with muscular 

 walls, by which their contents can be driven out. They appear, 

 in fact, to be used like the swim-bladder of fishes. 1 Many 

 Chaetopods take in water, by the anus no doubt for respiratory 

 purposes and pass it forwards along the intestine. In the 

 Capitelliformia a special groove conducts the water for some 

 distance, then the groove becomes closed to form a canal, which, 

 after a course forwards as a free tube below the intestine, again 

 enters the latter, constituting a " siphonal apparatus," similar to 

 that of the Echiuroids and the sea urchins. 



Sense Organs. In addition to the prostomial eyes, which are 

 present in nearly all the Nereidiformia and Spioniformia, eyes 



may exist elsewhere on the 

 body : thus Myxicola in- 

 fundibulum and Fabricia 

 possess a pair on the anal 

 segment; in M. aesthetica 

 Clap, there is a pair to 

 every segment ; in Uranch- 

 iomma there is a compound 

 eye near the tip of each gill 

 filament (i.e. palp) ; whilst 

 in Dasychone a series occurs 

 along each gill filament. 

 All these examples belong 

 A a to the Cryptocephala, in 



Fio. 143. A gill filament, A, of Branchiomma, which, Owing to certain 



peculiar modes of life, these 

 sense organs are required 

 in correspondingly peculiar positions. It is usually stated that 

 Polyophthalmw possesses, in addition to the usual prostomial 

 eyes, twelve pairs on as many successive segments ; but the 

 minute structure of these organs points rather to their function 

 as light-producing organs. 



The Capitelliformia and Opheliidae possess a pair of peculiar 

 " ciliated pits " or " nuchal organs " at the upper side of the head, 

 1 Eisig ; Mt. Zool. Slot. Ncapcl, ii. 1881, p. 255. 



