74 ARENICOLA ECAUDATA. 



and pit of the nuchal groove, but the first segment is probably united with the peris- 

 tomium behind, certain rings pertaining to the one and certain to the other. 



The arms of the nuchal organs or ciliated grooves commence ventro-laterally, and 

 run upward and backward, but, before meeting on the dorsal surface of the prostomium, 

 they bend sharply backward, and then unite in a short transverse line. The thickened 

 lips are nearly black and fairly wide apart, and thus contrast with the pink tint at the 

 bottom of the groove. In place of the cerebral lobes of A. marina there are several 

 posterior outgrowths of the commissure-like brain which resemble nerves. In some 

 specimens, however, a median process of the brain underlies the dorsal part of the nuchal 

 groove, which is present in the post-larval specimens of A. ecaudata 7*2 to 9*4 mm. 

 long. 



The brain is commissural in form, and merely unites the ends of the oesophageal con- 

 nectives. The ventral nerve-cord is ccelomic throughout in this and A. branchialis. Six 

 giant-cells occur in segments two to seven. Gamble and Ashworth insist that the giant- 

 fibres arise in all cases from giant-cells segmentally arranged. " The exact connection of 

 the giant-fibre with the cord, and especially the connection of its branches and of those of 

 the more inconstant lateral giant-fibres, require further investigation, but we (Gamble and 

 Ashworth) have shown that in A. branchialis the branches accompany the peripheral 

 nerves, and are connected by short offsets with the fibrous matter of the cord, thereby 

 increasing their likeness to efferent nerve-fibres." 



Ashworth, 1 in Arenicola cridata (Gamble and Ashworth, 1900, p. 443, and Plate 

 XXIV, fig. 33), describes small papillae or sometimes depressions corresponding in position 

 to those of the sense-organs of the Capitellida3, ScaMbregma, Ophelia, etc., but their 

 structure could not be demonstrated. 



Eisig thinks the sense-organs modified cirri, and that the foregoing would repre- 

 sent the dorsal cirrus of the ventral division. Ashworth disagrees, pointing out that the 

 feet of the Glyceridse are, though occasionally single, truly biramous. Eisig's contention 

 that such a foot in the Glycerids is equivalent to the ventral division only does not hold 

 good. 



Ten narrow rings occur in front of the first foot. 



Body (Plate XCV, fig. 10) about 6 in. or more in length, of a brownish iridescent 

 hue, darker at the snout and posteriorly. Some are of a very dark deep green colour, 

 and tinge spirit to a deep green hue. The branchiae are of a dull red, and on the 

 whole more conspicuous than in A: marina. They commence on the seventeenth bristled 

 segment, and continue, though smaller, to the posterior end, the last, however, being 

 represented only by a single papilla. So far as could be observed the gill is more finely 

 branched at the tips than in A. marina, but the stems seem to be proportionally fewer 

 and thicker, and to arise from a basal part, so that the character of the gill differs. 

 Gamble and Ashworth note that the stems bifurcate, and then the posterior becomes 

 dichotomous near its tip. 



In a post-larval form (7"2 mm. long) a linear series of twelve eyes occurred on each 

 sida of the prostomium. The anterior eyes are placed in the nervous layer and on the 

 course of the nerves from the front edge of the commissural brain to supply the pros- 



1 'Quart. Journ. Micr. Sci./ vol. xlv, n. s., p. 276. 



