80 A ren icolidae 



Post-Larval Stages of ARENICOLA ASSIMILIS var. AFFINIS. 



Mr. E. Vallentin found three worms on the surface of the sea, 

 near the Falkland Islands, which the writer 1 has shown to be 

 post-larval stages of A. assimilis var. affinis. The specimens were 

 7 6, 8 ' 7 and 11 1 mm long respectively, and were abranchiate. The 

 largest was provided with a transparent gelatinous envelope about 

 1 mm. in diameter, which covered the worm except for a distance of 

 nearly 1 mm. at each end. These specimens resembled post-larval 

 stages of A. marina, except that there were six to eight oesophageal 

 glands on each side (PI. X, Fig. 29). Six pairs of nephridia were 

 present, and on the last three the gonad was already recognisable. 

 The statocysts were found as two in v aginations of the peristomial 

 epidermis, the lips of each being approximated so as to form a short 

 tube. In each statocyst there were four to six statoliths sand- 

 grains or fragments of spicules. A gill-less post-larval stage of this 

 species and variety, 6 * 5 mm. long, was found among the " roots " of 

 seaweeds near Uschuaia. 2 These four are the only known post-larval 

 specimens of this species. 



Post-larvae of A. assimilis may be differentiated from those of all 

 other species of Arenicola, except A. pusilla, s by the presence of 

 several pairs of oesophageal glands. The presence of statocysts, 

 which was established only after careful examination, showed that 

 the specimens described above could not belong to the species 

 A. pusilla, but must be examples of A. assimilis, and, having 

 nineteen chaetiferous segments, they were referable to the variety 

 affinis. 



Post-Larval Stages of AEENICOLA ECAUDATA. 



The writer has examined about thirty specimens, a dozen of 

 which were abranchiate. The latter range in length from 3 to 

 9 '4 mm., and, in addition to the peristomium and the achaetous 

 body-segment, have fifty-three to fifty-eight fully formed and 

 chaetiferous segments, and two or three recently differentiated and 

 as yet achaetous segments, followed by the pygidium (PI. XI, 

 Fig. 34). The prostomium is large and bluntly conical, and bears 

 groups of eyes latero-dorsally. The achaetous peristomium contains 



1 Q. J. Micr. Sci., xlvi (1903), p. 764. 



2 Ehlers, E., " Polychaeten," in Hamb. Magalh. Sammelreise, ii, 1 (1897), 

 p. 104. 



3 Post -larval stages of this species have not yet been discovered. 



