254 Mr. A. Agassiz on the Young Stages 



drop off, and the articulations become less numerous and even- 

 tually be lost, as in the adult Nemerteans. 



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 3-V 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 temporary (?) arti- 

 culate 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 embryological 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 

 palseontological point of view. We find repeated in Annelids 

 the same striking coincidence between certain features only 

 embryonic in the present types, and 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. 0. C. Marsh, of New Haven, 

 which were all provided with bunches or single bristles 

 of these large, rough setae, 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 bristles, 

 and their order of appearance in the types we have thus far 

 examined, seem the only characters capable of general applica- 

 tion of any systematic value ; when a greater number of An- 

 nelids have been studied, the dorsal cirri, as well as the charac- 

 ters of the tentacles of the anterior part of the body, will furnish 

 us with valuable additional guides for classification in relation 

 to the rank of families and genera ; and, as far as we can make 

 use of them, they seem to coincide remarkably with the generally 

 received notions of superiority and inferiority of the principal 

 families current among the most accurate investigators of 

 Annelids. 



