256 Mr. A. Agassiz on the Young Stages of a few Annelids. 



Fig. 20. Young Spirorbis soon after its escape from the egg, having only 

 one tentacle developed, on the right, ti. 



Fig. 21 . Somewhat more advanced than the preceding figure, showing the 

 first trace of opercular tentacle. 



Fig. 22. Young Spirorbis, having three pairs of bristles ; somewhat older 

 than the preceding stage. 



Fig. 23. The anterior extremity of a specimen more advanced than tig. 22, 

 showing the first trace of bifurcation of the tentacles. 



Fig. 24. Anterior extremity of a still older specimen, in which the con- 

 trast between the opercular tentacle (^o) and the others {t^ — ^4) 

 becomes very marked. 



Fig. 25. Young Spirorbis, in which all the characters of the adult can be 

 readily recognized. Lettering as above. 



Fig. 26. Young Folydora having already lost the temporary bristles of the 

 anterior rings. 



Fig. 27. Somewhat older than the preceding figure ; the pigment-spots of 

 the anterior part are more marked : from below. 



Fig. 28. Older than fig. 27 ; the dorsal cirri are quite apparent. 



Fig. 29. Young Folydora, seen from belovy ; the pigment-cells are more 

 concentrated than in the preceding stages. 



Fig. 30. Somewhat more advanced : seen from the dorsal side : the pig- 

 ment-spots are reduced to mere dots ; the number of rings has 

 not increased from the preceding stage. 



Plate VI. 



Fig. 31. Head of Folydora having forty rings. 



Fig. 32. Anterior of young Folydora not quite full-grown ; from below. 



Fig. 33. Adult Folydora, seen from above. 



Fig. 34. Profile view of anal extremity of Folydora. 



Fig. 35. Profile view of the anterior extremity of Folydora. 



Fig. 36. Gland found at the side of each of the rings beyond the sixth, at 



the base of the dorsal cirrus. 

 Fig. 37. Stiff bristles of the fifth ring. 



Fig. 38. Small hook-shaped bristle of rings following the fifth. 

 Fig. 39. Young Nerine provided with the temporary bunch of bristles. 

 Fig. 40. Nerine in which the tentacles and dorsal cirri have begun to de- 



velope, which has lost the large bunch of serrated bristles. 

 Fig. 41. Somewhat more advanced than fig. 40. 

 Fig. 42. Young Nerine, having most of the characters of the adult. 

 Fig. 42^. Portion of one of the serrated bristles of the temporary cluster 



of fig. 39. 

 Fig. 43. Anterior extremity of an adult Nerine, seen in profile. 

 Fig. 44. Hook-shaped stiff bristles of the lower side. 

 Fig. 45. Posterior extremity of Nerine. 

 Fig. 46. Young Phyllodoce, from the dorsal side. 

 Fiff. 47 (upper fig.). Fig. 46 seen in profile. 

 Fig. 47 (lower fig.). The tentacles of the anterior part of the Phyllodoce 



have developed ; the body has considerably^ lengthened : seen 



from the dorsal side. 

 Fig. 49. The same, seen from the mouth side. 



Fig. 50. Young Phyllodoce, seen in profile in stage of fig. 47 (lower fig.). 

 Fig. 51. Somewhat older than the preceding stages; seen from above. 

 Fig. 52. Young Phyllodoce in which the body has greatly elongated ; seen 



from above. 

 Fig. 53. Head of adult Phyllodoce maculata, from above. 

 Fig. 54. Paddle and setae of adult of fig. 53 ; seen in profile. 



