On the Young Stages of a few Annelids. 339 



less numerous and eventually be lost, as in the adult Nemer- 

 teans. 



Another young worm, equally striking, is represented in 

 Figure 58 ; it is a parasitic Annelid, attached by its posterior 

 extremity to the underside of the carapace of lobsters, measures 

 about -g 1 ,,- of an inch in length, and consists of numerous rings ; 

 the mouth is edged by a series of small hooks. On the two 

 sides of the anterior part we find three large temporar}'- (?) ar- 

 ticnlate bristles, four or five times as long as the width of the 

 body, the middle bristle is the longest; next come eight rings 

 without appendages of any sort, the succeeding three rings are 

 each provided with a long bristle, similar to those of the ante- 

 rior extremity. These are the only appendages of the Annelid, 

 the numerous rings of the body being bare ; the anal extremity 

 is somewhat club-shaped. The digestive cavity was not as yet 

 subdivided into separate regions, and nothing in this young 

 worm, in spite of the great number of rings, indicated even the 

 family to which it might belong. 



Although the embryologieal data at our command will not 

 suffice in guiding us to any valuable systematic conclusions, yet 

 the presence of temporary bristles of huge size in the young of 

 so many Annelids is a feature of the greatest interest from a 

 paleontological point of view. We find repeated in Annelids 

 the same striking coincidence between certain features only 

 embryonic in the present types, and which were characters of 

 the adults in past geological times. I was particularly struck 

 with this coincidence when examining a series of drawings of 

 fossil Annelids kindly shown me by Mr. O. C. Marsh, of New 

 Haven, which were all provided with bunches or single bris- 

 tles of these large rough seta?, entirely out of proportion to the 

 width of the body, and similar to those found in the embryonic 

 Annelids we have noticed. The nature of the setae and bris- 

 tles, and their order of appearance in the types we have thus 

 far examined, seem the only characters capable of general ap- 



