344 Ralph S. Lillìe 



such as cilia retaining, in favorable preparations, almost the normal 

 appearance. 



Yoiiug larvae of the swarmiug- stage were best prepared by 

 fixation in Hermann's fluid for two minutes, followed by Merkel's fluid 

 for oue hour. It is neeessary to imbed such larvae as soon as possible 

 after tìxation; otherwise the yolk becomes so brittle as to render it 

 practically imi)0ssible to seeure unbroken sectious. If imbedded and 

 cut immediately after fixation, no such difficulty is experienced. 



The succeeding descriptions are based upon the examination of 

 a very large number of preparations, and ali possibility of decep- 

 tion arising from imperfect preparations, abnormalities of structure in 

 individuai larvae, or failure to observe criticai stages, has, it is 

 hoped, been avoided. 



3. General account of the larvai Development witli Especial 

 Reference to Segmentai Structures. 



Arenicola cristata, Stimpson, the species upon which the eutire 

 following study has been made, is found in several localities in the 

 neighborhood of Wood's Hole ■ — most abundantly at North Fal- 

 mouth, where the extensive sand-flats furnish ideal eonditions for 

 its development. The animals are found burrowing at a depth of 6 

 to 18 inches in the sand, which is deeply saturated with decaying 

 organic matter: and in the summer months are in nearly ali in- 

 stances well developed and of large size (6 — 12 inches in length). 

 For the larvai stages, it has proved neeessary to rely upon material 

 reared in the laboratory, for it has been found impossible in any 

 otber way to obtain the requisite quantity of young larvae. I bave 

 frequently searched the sand with a lens in the hope of finding 

 young Arenicolae, but always without success; the larvae, however, 

 lend themselves so readily to artificial rearing that no difficulty has 

 been experieuced in securing in this manuer an abundauce of speci- 

 mens of ali stages. 



The general characteristics of the adult A. cristata are sufficiently 

 known from the descriptions of Ötimps(jn, Verrill, Gamble & AsH- 

 woRTH, and others. The young free-swimming larvae bave been 

 briefly described by Wilson (Johns Hopkins Studies, 1883) and Child 

 (Z. Bull. Voi. 1, 1897). In the present paper I shall give merely a 

 short account of the general characteristics of the larvai stages, 

 omitting dctails that do not immediately bear on the problems under 



