POLYNOIDAE. 35 



POLYNOIDAE. 



In this family the body is flattened and narrow with the sides nearly parallel, 

 or else broader and oval and elliptic and in other cases elongate and more typi- 

 cally vermiform. 



Prostomium distinct, convex. Facial tubercle absent or weakly developed. 

 Eyes four. Two lateral tentacles and in most cases also a median tentacle 

 present. Palpi present, elongate. 



First somite, or peristomium, bearing usually setae in reduced number. 

 Two pairs of elongate cirri, the ventral cirrus of the succeeding somite also 

 ordinarily very elongate. 



Parapodia biramous with ventral cirri and dorsally in part notocirri and in 

 part elytra. Elytra borne on somites II, IV, V, VII and so on, on the odd somite 

 to the twenty third, after which, when elytra occur at all, two cirriferous somites 

 are ordinarily intercalated between two bearing elytra; more rarely the anterior 

 ones on II, IV, VI, VIII, and X. (Hemilepidia) . 



Setae all simple. Dorsal setae with simple or bifid and more or less hooked 

 tips. 



Pygidium with two anal cirri. 



Proboscis distally with a single circle of equal papillae; armed with four 

 horny jaws. 



The polynoids live under a great variety of conditions, some at considerable 

 depths, but the greater number in shallower water along the shores, among 

 Algae, growths of eel-grass (Zostera), bryozoans, and hydroids, such being 

 various species of Lepidonotus, Polynoe, Halosydna, etc. In contrast with the 

 sluggish movements of most, some swim freely and with considerable agihty 

 and gracefulness, e.g., Halosydna gelatinosa M. Sars. (Gravier, Nouv. arch. Mus. 

 hist, nat., 1901, ser. 4, 3, p. 206) and a few forms, such as Drieschia pelagica 

 Michaelsen, Nectochaeta, and Plotolepis, gen. nov., are pelagic. Many are 

 commensals, different forms living in association with sponges {e.g., Lagisca 

 hexactinellidae and Polynoe ewplectellae occurring in Eupledella aspergillum and 

 having a corresponding remarkable transparency); coelenterates (e.g., Polynoe 

 rutilans Grube, occurring on and matching in color an alcyonarian, Xenia); 

 starfishes, many polynoids placing themselves in the ambulacral grooves near 

 the mouth and apparently there securing fragments of food escaping the star- 

 fish {e.g., the well-attested case of Acholoe astericola Delle Chiaji occurring on 

 Astropecten aurantiacus (Linne), A. hispinosus (Otto), A. irregularis (Pennant), 



