114 On the Development 0/ Arenicola piscatorum. 



Limivora. 



Telethusa .. Arenicola, piscatorum. Sclmltze, in this paper. 



Terebellacea. Terebella nebulosa, kc. Milne-Edwards, Ann. des Sci. 



Nat. .3 ser. iii. p. 145. 

 Hermellacea. Hermella Quatrefages, Ann. des Sci. 



Nat. 3 ser, x. p. 153. 

 Serpulacea. . Protula Milne-Edwards, Ann. des Sci. 



Nat. 3 ser. iii. p. 161. 

 yjHj(/v Fahricia (Amphicora) O. Schmidt, Neue Bcitr. zur 

 '^ I sahella. Naturgesch. der Wiirmer, 



""' ' 1848, p. 27. 



Ch^topterea. Chatopterus M. Miiller, Arch. 1855, p. 1. 



Mesotrocha sexoculata, J. 



MuUer, Arch. 1846, p. 101 ; 



Busch, Mull. Arch. 184/, 



p. 187; Beoh. &c. 1851, 



p. 59. 



EXPLANATION OF PLATE II. 



Fig. \. Mass of eggs of Arenicola piscatorum, enclosed in a pedunculate 

 gelatinous envelope : natural size. 



Fig. 2. Embryo of Arenicola after the process of segmentation has reached 

 the stage in which the embryo commences its movements in the 

 gelatinous mass by means of a broad band of cilia at the anterior 

 end (about 10 days old). 150 diameters. .^ot>'<!fti::> 



Fig. 3. The same embryo, rather more elongated (one day older), a. »VA 



Fig. 4. An embryo in which the body has become more elongated, with 

 the appearance of new bands of cilia (12 days old). 



F'ig. 5. An embcyo, in the interior of which the diiFerentiation of the 

 central cord (the alimentary canal) has commenced, with the 

 appearance of the first traces of segments (13 days). 



Fig. 6. An embryo in which the intestinal canal and the segments are still 

 more distinctly developed; the general cavity is traversed by 

 transverse walls, corresponding in number with the segments, 

 and attaching the intestine to the inner surface of the walls of 

 the body : the circles of cilia are still unaltered (17 days). 



Fig. 7. A young larva, 24 days old, which has escaped from the gelatinous 

 envelope. The circlets of cilia have disappeared, the number of 

 segments has considerably increased, and the anterior extremity of 

 the intestine passes into a barrel-shaped oesophagus, the wide an- 

 terior opening of which is close to the mouth (a). The anterior 

 segments of the body are furnished with setae. 80 diam. 



Fig. 8. Anterior extremity of the same embryo, seen from the side to show 

 the position of the mouth on the ventral surface. 



Fig. 9. Setaj from the anterior segments of the body. 400 diam. 



Fig. 10. Young of Sacconereis, with four circles of cilia, from Heligoland. 



Figs. 11 & 12. Young of Nereis diversicolor, just escaped from the cavity 

 of the mother; fig. 1 1 , from the ventral side, showing the mouth, a ; 

 fig. 12, from the back. 



