1 It On the Development «/" Arcnicolu piscatcirmu. 



Limivora. 



Tki.ktiusa .. Arrvicnlo piscntonnn. Scliultze. in tliis jm]>pr. 

 TKiiKHKhLACEA. TerebcUa tiebulosa, &CC. Milne- Kdwanls, Ann. des Sci. 



Nat. .'5 sor. iii. p. 145. 

 IIkkmkm.acka. Hcriiiella Qimtrcfages, Ann. «lo8 Sci. 



Nat. '.i st'r. X. p. 15.3. 

 Skkpi'lacea . . Protiila Miluc-Kilwards, Ann. des Sci. 



Nat. .3 ser. iii. p. 1(>1. 

 Fttliricia {Amphicora) O. Scluuidt, Neuc lU-itr. znr 

 sat)ella. Natiugescli. dcr Wvirnier, 



IS JS. p. 27. 

 ("n.«TOPTEREA. Chtptopterus M. Miiller, Arch. 1H55, p. 1. 



Mesotroclifi sexocutafri, J. 



Miiller, Arch. lSJ(i, p. 101; 



Busch. Mull. Arch. 1847. 



p. 187; Heob. &c. 1851, 



p. 59. 



EXPLANATION OF PLATE IL 



I'iy. 1. Miiss of eggs of Arenicola piscatorum, enclosed in n j)cdunculate 

 gelatinous envelope : natural size. 



Fig. 2. Enihryo of Ar<-tiicoln after the i)rocessof segmentation has reached 

 the st.age in which the emhryo commences its movements in the 

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

 end (about I'l days old). 150 diameters. 



Fitf. .3. The same embryo, rather more elongated (one day older). 



Fi(/. 4. An embryo in which the body has l)ecome more elongated, with 

 the a])pearance of new bands of cilia (12 days old). 



Fir/. 5. An embn,'o, in the intenor of which the differentiation of the 

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

 a])pearance of the first traces of segments (13 days). 



Fig G. An embryo in which the intestinal canal and the segments arc still 

 more «iistinctly 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 iana, 24 days old, which has escaped from the gelatinous 

 envelope. The circlets of cilia have disa])peared, the number of 

 segments has considerably increased, and the anterior extremity of 

 the intestine ])asses 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 funiished with sct»c. 80 diain. 



Fig. 8. Anterior extremity of the same eml)ryo, seen from the side to show 

 the position of the mouth on the ventral surface. 



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



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



Figs. 11 & 1 2. ^ Oung of Nereis diver. s-icolor, just escai)ed from the cavity 

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

 fig. 12, from the back. 



