BRISTLE-WORMS 429 



There are no auditory organs, and the four simple eyes have 

 already been mentioned. 



The Bristle -Worms are subdivided into three orders: — i. 

 Many -bristled Worms (Polychaeta); 2. Few-bristled Worms 

 (Oligochaeta); and 3. Simple Segmented Worms (Archiannelida). 



Order i. — Many-bristled Worms (Polychaeta) 



This order embraces an enormous number of marine worms, 

 possessing a considerable number of seta; and agreeing in many 

 other particulars with Nereis, which is a type of the order. Over 

 thirty families are recognized, grouped into seven sub-orders, but 

 it will be sufficient for our purpose to divide the order into two 

 groups: — I. Free-living Polycheetes (Errantia), and 2. Sedentary 

 Polychaetes (Sedentaria). 



1. Free-living Polychcstes (Errantia), of which Nereis is a 

 good example, are carnivorous forms which move actively about 

 in pursuit of their prey, swimming, crawling, or burrowing. To 

 this habit their structure corresponds, for the locomotor organs 

 (foot-stumps) are well-developed, as also is the head-lobe, with 

 its eyes and feelers. The pharynx can be protruded in the way 

 described for Nereis and is usually armed with horny jaws. 



Only one or two British forms can be mentioned here in 

 addition to Nereis. Species of Polynoe are common on our 

 shores, and these can readily be identified by their elongated 

 oval form and the presence of a double series of scales (elytra) 

 on the upper surface, which appear to be organs of respiration. 

 Built on somewhat the same lines is the conspicuous "Sea- 

 Mouse" {Aphrodite), the plump body of which attains a con- 

 siderable size. The setae are of several kinds, and very numerous : 

 some of those arising from the upper divisions of the foot-stumps 

 are much elongated and beautifully iridescent. As in Polynoe, 

 there is a double row of elytra, but they are here covered by 

 a tough membrane consisting of numerous small setae matted 

 together. 



2. Sedentary Polychcstes (Sedentaria) either have limited 

 powers of movement, and inhabit permanent tubes, or else are 

 burrowing forms, which pour out a sticky fluid from the skin that 

 glues together the sand or mud surrounding them into a sort of 

 temporary case. They live upon vegetable matter. The head- 



