146 NERINB FOLIOSA. 



After the fiftieth, foot l a change takes place in the nature of the bristles. The 

 bristles of the upper division remain for the most part similar to those in front, though 

 the row is longer, but those of the ventral row, with the exception of a few interiorly, 

 have assumed the form of the hooks characteristic of the posterior region, viz. stout 

 shafts deeply inserted in the tissues, a stout distal part slightly tapered and ending in a 

 blunt tip, on each side of which is a wing (Plate CII, fig. 7 b and 7 c). 



At the seventieth foot the branchia has lost much of its external membranous frill, 

 and is again separated inferiorly from the posterior fillet of the upper division of the 

 foot which rises into a prominent border superiorly. The anterior fillet has disappeared 

 in both divisions, and the fillet from the second ring of the segment runs up behind the 

 posterior fillet at its ventral edge. The bristles in the upper division remain simple, but 

 are more slender. Those of the inferior division consist of winged hooks, with the 

 exception of a few superiorly and inferiorly. 



At the ninetieth foot (Plate XCVII, fig. 1 b) the branchia has still further diminished, 

 the upper (posterior) fillet is prominent, the lower ends in a free peak superiorly and 

 inferiorly, and the rows of bristles are shorter. The superior division still shows the 

 tapering bristles, but amongst them are a number of the winged hooks. The notch 

 between the divisions is deep, and the lower has only a few of the slender tapering bristles 

 superiorly and inferiorly, a short row of powerful hooks projecting between them toward 

 the lower part of the division. 



The chief changes toward the posterior end are the diminution of the branchia, which 

 at the one hundred and twenty-fifth foot (Plate XCVII, fig. 1 c) forms a process which is 

 not equal to the vertical diameter of the upper fillet, the decrease in the number of the 

 superior bristles which are in a single fascicle, the abbreviation of the upper border of 

 the long fillet of the ventral division, and the increase and prominence of that part of 

 the fillet bearing the bristles and hooks. The hooks are the chief feature — both dorsally 

 and ventrally — forming a short row in the upper and a long row in the lower division. 

 They are of great strength, have a slight curvature at the diminished tip, and a wing at 

 each side. The tip appears to be broken in many, so that slight complexity exists at the 

 end of the forward curve distally. The bristles occur above and below each row of hooks, 

 are few in number, and have the same form as in front. 



Still further backward the branchia has shrunk to a minute rounded papilla, the 

 upper fillet is short and almost semi-circular, bearing very long hooks and a long slender 

 bristle or two above and below, whilst a broad gap separates it from the inferior fillet, 

 which, though diminished, resembles that in front, viz. has a more prominent margin at 

 the lower half which bears the hooks, with a bristle or two toward the upper and lower 

 margins of the row. The lower margin of the fillet is more prominent than the upper. 



The circulation is less intricate than in Magelona. The bright-red blood is driven 

 forward from the tail in the dorsal vessel, and sends off a branch in each segment to the 

 branchia, which, after aeration, passes its blood into the trunk joining the ventral. At 

 the tail a kind of caudal reservoir is formed at the junction of the trunks. 



1 Mesnil says the hooks appear ventrally on the fifty-eighth segment and dorsally on the sixty- 

 fifth. He gives the number as twenty ventral and twelve dorsal. In the early rows of hooks the 

 anterior range of bristles persists, but they by-and-by disappear. 



