Caudate Species of Arenicola 83 



The Caudate Section of the Genus ARENICOLA. 



The characters of the caudate section may be given thus : The 

 parapodia and gills do not extend to the posterior end of the worm, 

 an achaetous and abranchiate " tail " is present. There are eleven 

 or thirteen pairs of gills, the first borne usually on the seventh * or 

 eighth 2 segment; the gills, the axes 3 of which bear branches 

 bilaterally, are either pinnate or fruticose (bushy). The pro- 

 stomium is trilobate. Each statocyst 4 is either provided with 

 a tube leading to the exterior, in which case numerous statoliths, 

 generally of foreign origin, are present, or it is a closed vesicle 

 containing only a single secreted statolith. The dorsal lip of each 

 uephridiuin bears a fringe of flattened vascular processes; the 

 margin of the ventral lip is not deeply notched, it may be thrown 

 into folds or frills, but it is more often simple. The gonad is small, 

 the ova are biconvex, 5 and their vitelline membrane thin (1-3 //,). 



KEY TO THE CAUDATE SPECIES OF ARENICOLA. 



1. Nineteen or twenty chaetiferous segments ; thirteen (or 



twelve) pairs of gills, the first on the seventh or eighth 

 segment 2. 



Seventeen chaetiferous segments ; eleven pairs of gills, 

 the first on the seventh segment 6. 



2. Neuropodia of the posterior branchial segments long, their 



grooves extending almost to the mid-ventral line ; one 

 pair of oesophageal glands ; one pair of septal pouches 3. 



Neuropodia of the posterior branchial segments short, 

 forming oval pads on the sides of the segments, their 

 grooves not nearly reaching to the mid-ventral line; 

 several pairs of oesophageal glands ; no septal pouches 4. 



3. Lobes of the prostomium almost equal in size ; nephridia 



opening on the fourth to the ninth segments ; septal 

 pouches small ; statocyst s each with a tube leading to 

 the exterior, and with numerous statoliths (sand-grains) marina, p. 86. 



Median lobe of the prostomium large ; nephridia opening 

 on the fifth to the ninth segments ; septal pouches very 

 large; statocysts closed, each containing a single large 

 secreted statol th lovenif p. 108. 



1 The gill of this segment may be reduced or absent. 



2 In typical examples of A. assimilis. 



3 In A. glacialis the gill-axes are much reduced and the branches clustered. 



4 Statocysts are not present in A. pusilla. 



r> In those species in which ova have been examined in the fresh condition. 

 The ova of two species have not been available in the fresh state. 



6 The notopodial chaetae of this species, with their well developed " Sage- 

 blatter" and transverse striation, are very characteristic (see p. 44). 



G 2 



