SEA-FISHERIES LABORATORY. 277 



characters of the former are — the prostomium is distinct 

 and often bears, in addition to the paired eyes, sensory 

 tentacles and palps (as, for example, in Nereis) ; the body 

 segments are more or less alike and not divisible into two 

 sharply-marked regions, distinguishable by the different 

 arrangement and character of their setse and by internal 

 differences. In the Cryptocephala the prostomium is 

 compressed or hidden by forward growth of the peris- 

 tomium and thus becomes insignificant; the tentacles are 

 reduced but the palps become greatly enlarged and sub- 

 divided, forming the crown of gills ; the body is divisible 

 into a " thorax " and " abdomen," distinguished by the 

 form and arrangement of their setee and by certain 

 internal differences. Arenicola belongs to the branch 

 Phanerocephala which is divided into five sub-orders. 



I. ZSTereidiformia. — Prostomial tentacles and palps 

 well developed ; peristomial cirri usually present ; para- 

 podia well developed with acicula, dorsal and ventral 

 cirri ; seta? usually jointed ; muscular pharynx with 

 chitinous jaws; septa and nephridia regularly repeated 

 throughout the body. Chiefly predaceous and carni- 

 vorous worms, e.g., Nereis. 



II. Spioniformia. — Prostomium without tentacles 

 and palps ; peristomium with a pair of long cirri ; para- 

 podia project only to a slight extent, their dorsal cirri may 

 be large and act as gills, seta? unjointed ; no jaws; septa 

 and nephridia regularly repeated. Chiefly tubicolous or 

 burrowing worms. 



III. Terebelliformia. — Prostomium forms a promi- 

 nent lobe (upper lip) with or without tentacles, but with- 

 out palps ; peristomium may bear cirri or " tentacular 

 filaments"; parapodia feebly developed, ventral cirri 

 absent, dorsal cirri may form gills, setse unjointed, uncini 

 (short, sharply-curved dentate hooks) usually present; 



