462 THE ANNELIDA POLYCHAETA. 



the first six, which are separated on the ventral side by intersegmental furrows. 



The prostomium is obliquely truncate. Above it bears on each side a 

 transverse series of stout, golden yellow spines, or paleae. At the caudal border 

 of the prostomium above is a marginal raised ridge, which is entire, or sometimes 

 crenulate. Below the paleae a very characteristic, transversely placed, mem- 

 brane or velum, the margin of which is commonly frayed or incised into cirriform 

 processes, more rarely entire, this cephalic membrane, or limbus, overhanging 

 the mouth. At each side of the mouth a group of numerous filiform tentacles 

 which are not retractile within the mouth. 



On each side of the head two filiform tentacular cirri, one of which arises 

 near the ectal end of the series of paleae, the other inserted on the succeeding 

 somite; or these perhaps sometimes absent (Scalis). 



There are two pairs of branchiae, two inserted on each side on what appear 

 as the third and fourth obvious somites, or three pairs may be present. These 

 branchiae are lamellar, with anterior border finely and densely pectinate. 



Ventral glands weakly developed, limited to the first few somites. 



Aside from the paleae there are two kinds of setae, the capillary notopodials 

 and the unciniform neuropodials. The capillary setae form fifteen, seventeen, 

 or eighteen pairs of fasciae; they are limbate and curved, part of them showing 

 a finely serrulate border toward the distal end. The neuropodial setae are 

 acicular crochets, the free distal margin pectinate, with often part of the teeth 

 much smaller than the others, and the base prolonged into a stalk, not squat 

 plates such as occur in the ampharetids; they begin ordinarily on the third or 

 fourth setigerous somite. 



Nephridia apparently variable in number from species to species. 



Alimentary tract showing no such straight, wide stomach as occurs in the 

 Ampharetidae, and never with anterior, or internal, caeca. 



The ampharetids always inhabit tubes which they never normally leave 

 and which they are unable to reconstruct when they have been removed. These 

 tubes are somewhat conical in form and very regular, and are open at both 

 ends. They are either straight or somewhat curved. The walls are composed 

 of grains of sand, mud, fine shells, or even sponge spicules cemented together 

 with mucus and lined with a thin membrane, and are of astonishing regularity. 

 {Cf. Fauvel, Mem. Pontificia accad. Rom. Luicci, 1903. 21, p. 7). The small 

 end of the tube is normally uppermost, projecting above the surface of the sand, 

 with the large end down. The animal's head is toward the large end, which is 

 closed by the paleae fitting something like an operculum. The worms use 



