740 J. H. ASFIWORTH. 



II. Arenicola assirailis, Elilers. 



The two complete specimens found at low-tide mark at 

 Uschuaia are 105 mm. and 120 mm, long respectively. The 

 specimen from Punta Arenas would have been about 120 nun. 

 long if complete. Their colour in spirit is a fairl}' nniform 

 yellowish brown, but the tail of one specimen is of a some- 

 what darker colour. The body is slightly swollen in the 

 anterior region. The animal closely agrees in form Avith 

 specimens of A. marina of the same size. 



External Characters. — The prostomium (see fig. 20) is 

 moderately developed; the two lateral lobes are in the form of 

 a V, the arms of which embrace tlie median lobe.^ The 

 prostomium resembles that of A. marina and A. cristata, 

 except that the median lobe is proportionately smaller in 

 Ehlers' species. The nuchal organ is similar in its structure 

 and relations to that of A. marina. The metastomial 

 grooves indicating the track of the ccsophageal connectives 

 are well marked. 



T^here are twenty cliEetigerous segments_, the last thirteen 

 of which are branchiferous ; the first gill is thus on the 

 eighth chfetigerous annulus. In this character A. 

 assimilis differs from all other caudate Arenicolidje, in 

 which the first gill (except in those abnormal cases in which 

 the first true gill is missing) is on the seventh cha3tigerous 

 annulus. Of the throe specimens in my possession only one 

 has thirteen pairs of fully developed gills; in another the first 

 right gill is very minute ; while in the third specimen the first 

 left gill is small, and the last gill on each side is considerabl}' 

 smaller than the preceding one. Ehlers (1897, p. 104) records 

 specimens with only twelve pairs of gills. 



The gills of the specimens from Uschuaia are dense bushy 

 structures resembling the dendritic type of gill found in 



* There is a rougli sketch ("d'apres Elilers") of tlie prostomium of A. 

 assimilis in a memoir by Fauvcl (1899, p. 178), which, however, docs not give 

 an accurate imi)ressiou of its form, as the two lateral lobes are represented as 

 separate, whereas they are actually united posteriorly to forDi the V-shapi-d 

 structure described above. 



