252 Mr. A. Agassiz on the Young Stages 



the little Phyllodoce has the shape of fig. 53, and the broad 

 flappers resemble fig. 54, while the anal cirri have nearly assumed 

 the shape of fig. 55. 



From the earliest stages, the larva is never very transparent ; 

 it is distinctly tinged with brown, becoming darker with ad- 

 vancing age, till, in the stage of fig. 52, marked spots darker 

 than the main colour appear on the median line, which gives 

 these young Annelids such an unmistakeable resemblance to an 

 adult PJnjllodoce well known to me, and which I had always 

 identified as Phyllodoce maculata, Oerst., that I scarcely needed 

 the additional proof of raising these young to the full-grown 

 Phyllodoce to convince myself of their identity. 



On the Types of Development in Annelids, 



Several attempts have been made to classify the larvae of 

 Annelids. Busch*, Miillerf, SchultzeJ, and Claparede§ have 

 endeavoured to reduce the forms observed to a few general types. 

 Claparede has fully demonstrated that neither the classification 

 of Busch, Miiller, nor Schultze will satisfy the facts thus far 

 observed ; and it seems probable that Claparede's classification 

 must share the same fate : we should only remember that all 

 these attempts are based upon such few and incomplete ob- 

 servations that we cannot expect them to apply to subsequent 

 discoveries. The features used by Claparede to form his sub- 

 divisions seem somewhat objectionable, as it would be impos- 

 sible, unless the complete development of the larva were known, 

 to ascertain to which of his divisions they belong; and yet these 

 characters are, as far as the development of Annelids is now 

 known, the best that have been proposed. The presence of 

 temporary bristles is a good criterion for one division, and ap- 

 pears to be connected with fundamental differences in the larvae, 

 though the other division, based upon their absence, is liable to 

 the usual objections to characteristics derived from negative 

 features alone. How much more remains to be done before 

 any such classification of the Annelid larvae can be attempted 

 with the least chance of success is best shown by examining in 

 any tabular view the number of families of which we know no- 

 thing as yet of their embryonic conditions. And though (since 

 the time of the first papers by Loven, Sars, Milne-Edwards, 



* Beobachtimgen, p. 55. 



t "Ueber die Jugendzustande einiger Seethiere," in Monatsb. d. Akad. 

 der Wiss. zu Berlin, 1851, p. 422. 



J " Ueber die Entwickelung von Arenicola piscatorum, nebst Bemer- 

 kiingen iiber die Entwickelung anderer Kieraenvviirmer," in Abhand. Nat. 

 Ges. zu Halle, iii. 1855, p. 213. 



§ Beobachtungen, p. 84. 



