///. ANNELIDA: CH^ETOPODA. 313 



Nearly all Polychsetes are dioecious and undergo a more or less pro- 

 nounced metamorphosis ; with few exceptions (Manyunkia * from the 

 Schuylkill, Nereis * in California) they are marine. They are usually 

 divided according to their habits into fixed (Sedentaria) and free forms 

 (Errantia), but this classification lacks a morphological basis. The Seden- 

 taria feed on vegetable matter, usually form tubes of leathery organic 

 substances, in which foreign matter may be incorporated or which may 

 be calcified. The worms project their anterior segments from the tubes. 

 The Errantia often secrete gelatinous tubes in which they can hide, but 

 they never lose their powers of locomotion, and from time to time leave their 

 retreats and swim about preying on other animals. Correlated with habits 

 are differences in structure. In the Errautia the head and trunk are not 

 very different ; the anterior part of the alimentary tract can be protruded 

 as a proboscis, and, corresponding to their predaceous habits, is often 

 armed with strong jaws (fig. 281, A). In the Sedentaria there are no such 

 pharyngeal teeth, but, on the other hand, there is a greater difference 

 between anterior and posterior somites. In the 

 latter the parapodia are weakly developed, and 

 this part resembles the OligochaBtes in ap- 

 pearance. The head and beginning of the 

 trunk (thorax) are richly provided with gills 

 and tentacles for respiration and the taking of 

 food (fig. 282). 



Sub Order I. ERRANTIA. Predaceous 

 annelids with strongly armed pharynx. The 

 EUNICID^E, mostly represented on our shores by 

 small species, contains some species a yard in 

 length. Diopatra,* Nothria* The ALCIOPHXE 

 are transparent pelagic forms with large, highly 

 developed eyes. The SYLLIDJS usually have 

 three long tentacles ; Autolytus* Myrianida* 



(p. 310). The POLYNOHXE * (Lepidonotus* Poly- projecting at the sides. 

 noe* (fig. 283), Aphrodite aculeata,* the sea 



mouse, 6 inches long) are bottom forms with elytra covering the back. 

 NEREIDS ; Nereis virens,* the clam worm of all northern seas. 



Sub Order II. SEDENTARIA (Tubicola, Cryptocephala). These forms 

 cannot wander about at will, but live in their tubes. In the SABELLI- 

 DJE the tube is membranous and there is a crown of gills ; Myxicola* 

 Chone,* Manyurikia* In the SERPULHLE the tube is calcified and is 

 closed by an operculum on one of the gills. Hydroides; * Spirorbis,* 

 forming coiled tubes on seaweed ; Protula* The ARENICOLID2E,* which 

 burrow in sand, have gills on the sides of the body. The MALDANID^E 

 (Clymene* Axiothea*) have extremely long segments and build tubes 

 of sand. The TEREBELLID^E (Terebella* Amphitrite (fig. 282), Thelepus*) 

 have numerous filiform tentacles and branched gills on the anterior end. 



The AECHIANELLID^], which lack bristles and parapodia, 

 must be placed near the Polychaetae and are usually regarded as very 



